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When asked how old I am, I know the answer more often than not. However, I usually double-check that what I am going to say actually is the correct answer. How? By doing the mathematics in my head. In other words: I subtract my birth date from the current date and round to next lower digit. So, nothing mysterious about this, right? Why do I even bring this up? Well, although I do know the very simple mathematics behind this answer, and although everyone seems to think this is a simple question with a simple answer, I actually beg to disagree. I hold that the expected simple answer (number of years) represents only one way of construing and answering this question. In essence, I think that there are at least ten ways of interpreting this question, and each way echoes a profound fact about us, about who we are, and even about the universe.

One way to answer this question is what I call the official answer, namely one’s biological age. The other nine ways can be aligned along hierarchical categories of identity. Each category widens the concept of individuality one more step. By identity I am referring to the who actually is the “I” in “How old are you?”

Let me sketch the categories of my answers before discussing the answers themselves in detail. Category one is the afore discussed traditional way of answering the question.Answers two to five lay on the same time scale: they pertain to the categories of psychological identity (who is the one being conscious?) and that of cellular age (the “I” as a multi-cellular organism). Answer six is tied to the question of whether we as individuals really exist, or whether we rather represent a collective (tribe, ethnic group, …), that usually stretches up to several ten thousand of years back in time. Answers seven and eight relate to the fact that we all belong to the same species, and that all life on earth is actually is kin. Here I will look at the tree of life as a symbol for this category, and the reader will be invited to join me on a dizzying journey through geological time. The last two answers address the “I” as the “stuff” we are made up, i.e. matter. In order to discuss these last two categories, we will even have to leave earth.

As I said before, category one is identical to the traditional way of construing the question of how old one is. The answer is the number of years that have passed since one’s birth. Let us get to category two. Here we start with casting doubt on the rather simplistic model of identity that underlies the first category. For whom is one referring to when asking this question. One way of construing this question is that one views the person asked as a conscious being. However, this assumes average patterns of consciousness, otherwise one’s official (category one) and one’s emotional age might not be aligned at all. Just think of being in a coma for a considerable time. Coming out of the coma one experiences a severe dissonance between one’s official chronological age and one’s experienced age: where did all the lost years go? Granted, few of us have been in a coma or will do so in the future, so this interpretation might seem a bit strenuous. But if we generalise this example and claim that our emotional age is related to a continuity of us being the same, a conscious individual, then this reservation about who one is, is anything but academic. Think for example of someone who suffered from severe depression for years and who has finally been cured. There is ample indication for depression being accompanied by a bias toward negative memories, and it might even be that “depression is associated with false memories of negative material.” In other words, not only seem depressed people to have a tendency toward remembering negative events and facts, but they even do get these negative events and facts wrong. So, once a person is healed and discovers these fact about herself, does she still feel that she is a continuation of the past “I”, that she actually is identical with the integral of all these negative and false memories? Or does she feel like a new person who recently woke up from a long, disturbing sleep? If the latter is the case, we might talk about many years lost, and in any case, subjective and official age are not longer aligned.

I agree that so far I my arguments have been rather exclusive, since only few go through the above described bouts during their lifetime. So, my discontent with the question about how old one is seems to be exaggerated. True, but are states during which our consciousness disappear really that rare? What about dreamless sleep? Where is the “I” when one sleeps? If we equal the “I” in the question of “How old are you?” with the conscious self, everyone is actually much younger than her biological age.

For answer three let us stay with the category of identity = consciousness for a bit longer, namely with the question when consciousness actually arises. It is custom to define our age as the time that has passed since our birth, but that tacitly implies that one is not conscious before being born. What about the nine months of gestation? Some evidence points toward that the foetus is asleep in the womb while its brain matures, and that birth is the first time for it to wake up and to thus become conscious. However, our knowledge concerning this question is so far rather limited, so concerning this answer I think we have to wait for more facts to come in. Interestingly, in East Asia age is actually counted from gestation (well, the calculation is a bit more involved), so answer three is anything but academic.

For the next category (answer four), let us cast the question of who we are in terms of what we are made of. One answer is obviously: cells! So the question of how old one is becomes a question of how old one’s cells are. The reader might remember from school that neurons do not divide after birth, and despite what has been reported in newspapers over the last decade, this is actually true for most of one’s neurons. So, one’s neurons are mostly of the same age as one’s official age, but that is not true for every cell in the body. First, everyone grows after birth, so the majority of cells enter the picture after birth, but even that picture is not fully correct, because while some cells do not die or at least live many many years, some survive only a couple of days. When we picture ourselves as a multicellular living organism, the age question has thus to be much more specific. Here the reply can be: “What cell type did you have in mind?” But even that answer is oversimplifying things, since there are about ten times more bacteria living in and on us than our own cells. So the question could be: “The age of me or my tenants?”

Fair point, the reader might admit, all our cells and our tenant bacteria age at different paces, but what about the one cell we were created from, the zygote? Is not this one cell the beginning of an individual, at least on a cellular level? This brings us to answer five. At a first glance, the zygote indeed produces a single age to the question of how old one is, but even then the question is more complex. This is because the age of the ovum and the sperm that become the zygote are of very different age. While one’s father’s sperm was created around the time of one’s conception, the egg was produced before one’s mother’s own birth. So, half of one’s DNA is actually older than even one’s mother.

Since I already brought parents into the question, let us increase the scope for the next answer, which also opens a new category: individual versus group. What is the next larger unit of us? Definitely a family, so here we gain some more years by bringing our still living ancestors into the equation. This seems a rather specious move, since we are definitely not our family, we are individuals! But is that really so? We are a social species, which not only means that we strive to be together and that especially our brain is highly adapted to interaction with others. It also means that the concept of the individual itself is rather specious. Just think of the following: what would one be without all the skills that were transferred to each of us from previous generations? These are skills that every one of persistently uses when interacting with others, and not only then. Without these skills no one could read this text or even write it. Further more, we would not even be privy to many of the abstract concepts that are the fruits of centuries of social evolution: individual (!), freedom, liberty, rights, etc. So whom is one talking about when asking: “How old are you?”

From that perspective one’s age becomes that of the group to which one belongs. One of the candidates for such a group is culture: the “I” as in “I am an Australian Aborigine.” So, the question is: how old is the culture that lends one this identity? It depends, but no matter how liberal one interprets this topic, the answer is usually some few thousands years. Even if is one hyper-liberal and maintains that a lot of, for instance, the Aboriginal culture was formed when their ancestors came to Australia, one still can not claim more than some ten thousands of years when answering the question of how old one is.

This upper limit brings us to answer seven. Behavioural modern humans, namely those that rely on symbolic thought for the expression of cultural creativity, emerged about 50 000 years ago. This distinction from other types of humans, namely “behavioural”, is fitting since this reliance on symbolic thought kick-started the rapid increase of culture in the lives of humans. And as I just discussed, culture is what makes us “us”.

But one can of course cast the net a bit wider and draw the border of who we are at the species border. After all, we love to differentiate ourselves from other apes. The age of our species is rather blurred, since the number of human fossils is actually astonishingly small. The current age estimate for the human species is about 200 000 years, which would be one version of answer seven to the question “how old are you?”

We have now reached our species’ border, and one way to continue would be to travel along the tree of life further back in time. That brings us to answer eight. Here the reader might want to stop this voyage, for what have other species to do with one’s age?? Well, that is what common descent is about: we share something with every living being ever alive, and that something is common DNA, and a hereditary tree along which this DNA was passed down to us. When we look at the sequence homology of DNA, we share 60% of genes with fruit flies (yes, fruit flies), and still 7% with bacteria. If we remind ourself of how far back in time we shared an ancestor with, for instance, a fruit fly, the motion that DNA is a time capsule very much comes to life. And if one perceive oneself as a mere container for DNA that is to be passed on, then it is not far fetched to equate one’s age to that of how far back in time one’s genetic ancestors lived.

So let us then take a travel back in time along the tree of life. The genus Homo is about 2.3 million years old, and we are the only surviving species of that genus. The further up one goes in the taxonomic hierarchy (species, genus, family, order, class, …) the further back in time one travels, and the less familiar our ancestors become. If we consider what distinguishes us from, for instance, a cat, this time journey takes us as far back in time as the origin of the order of primates. The most recent common primate ancestor lived around 60 million years ago. And on we go, further and further back in time. One important intermediate stop has to be made at the most recent common ancestor of Mammalia, which lived 210 million years ago. After all, being warm-blooded is something we all heavily identify with. Just think of the reprimand “What a cold person she is!”

There are some more steps to take (for example the appearance of the kingdom of Animalia around 600 to 700 million years ago) before one arrives at the universal common ancestor of all life on earth. This is a single-cellular organism that is believed to have lived 3.5 to 3.8 billion years ago. It is important to point out that while there is quite substantial evidence for the existence of a universal common ancestor, there are still dissenting voices in this debate. However, even if one does not believe in a universal common ancestor, there is still something every living being has in common, namely that it is animated and thus not inert. So let us talk about the origin of life. Naturally, the origin of light is still shrouded in uncertainties, and it is not even entirely clear whether life on earth maybe originatde from Mars, but the common census is that life on earth began around 3.6 to 4 billion years ago, so around the time when the hypothesised universal common ancestor lived, namely 3.5 to 3.8 billion years ago.

One can of course counter that everything I have said about culture, ancestry, and DNA is rather symbolic and cannot be taken for one’s true age, for everyone alive is a very late representative of each of these groups. While it is indeed true that every living being on life looks back at about 3.5 billion years of ancestry, this does not mean that the individual itself is equally old. Well, at least if one is a firm individualist and defines oneself by the outer layers on one’s epidermis.

Even if I consent this point, I think the next category is going to be very convincing, since it again addresses us as individuals. Here the question is of how old the “stuff” is of which we are made. This takes us to answer nine.

We are made of the same “stuff” than everything substantial around us: matter. All matter in the solar system stems from the gravitational collapse of a galactic nebula, an event which is estimated to have taken place around 4.5 billion years ago. So, the vast majority of all material we are made of dates at least 4.5 billion years back, and most of it actually even further. For where did the material of this galactic cloud come from?

Well, as the reader might have heard, we are all -mostly- star-dust. Every atom in the solar system that is heavier than lithium was synthesised in the nuclear furnaces of big stars that existed before the advent of the solar system and that dispersed major parts of their material into interstellar space. All that happened during events called supernovae. In other words: to a large degree we are reorganised star dust. The matter we are made of was once inside a star, and what is almost equally intriguing is that any two atoms in one’s body do not need to come from the very same star, so some atoms in one’s left hand most certainly originate from a different star than some in one’s right hand.

When did the stars, from which our solar system originates, exist? This is unknown, and it is also not clear how many stars contributed to our solar system. However, the formation, life, and death of these stars took place after the formation of the Milky Way, which is estimated to have taken place about 12 billion years ago. One might get the impression that our galaxy has hardly been around long enough to have produced the heavy elements we consist of, since main-sequence stars, to which our sun belongs, have an average life time of about 10 billion years. But it turns out that the heavier stars, which actually produce carbon and all the elements heavier than carbon, have much shorter life times of only some ten of millions of years, or even shorter. So, the answer to the question how old one is could thus be: “The heavier side of me is more than 4.5 billion years old.”

Which leaves us with the last layer of answer to this question, namely answer ten. So far I have only addressed the heavier elements, which stem for stellar nucleosynthesis, but what about the lighter ones, in particular hydrogen? Hydrogen is the simplest element of all known elements, and all hydrogen in the universe was created during the aftermath of the Big Bang. Since hydrogen atoms are abundant in organic molecules and, of course, the water in our body, a sizable portion of humans, namely 10% by weight, is almost as old as the universe itself. In other words, one’s lighter parts is over 13 billion years old. And if one wants to be very anal, one could argue, that the heavier elements in our bodies were produced from exactly this hydrogen, so ~ 13 billion years is the true age of the material we are made up.

But wait, who says there was nothing before the Big Bang, and that the building blocks of matter were not created during earlier times? Well, while there are many theories about earlier states of the universe, there has not been any successful experimental test of these theories yet, and it remains to be seen if one ever finds an experimental handle for rigorously testing such theories. We actually might have too few universes for answering the question about what was before the Big Bang.

If one thus wants to safely remain on the side of sound justification, the age of the universe is the largest age one is allowed to provide as an answer to the apparently rather simple question “How old are you?”, which, as we have seen, readily can cast the questioner into a journey of epic dimensions.

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Update on 2014-08-17: I added the concept of the East-Asian age to answer three.