You may think you know how old you are. But I'm about to tell you that you're wrong. You're wrong. There, see? I told you.
There are a few different ways to measure how long you've been alive. Depending on which man-made concept you use, the results vary wildly...
The Korean Ageing System
In the Western world, you celebrate your first birthday after you've been expelled from your mother's womb for a whole year.
But other cultures have a different set of rules.
In East Asia, the traditional method of ageing is to turn one year old on the day you're born. This actually makes good sense because your heartbeat, brainwaves and respiration have been activated in the womb for most of your gestation. Why not round it up to a year?
But then it gets weird.
In this Eastern system of ageing, your second birthday occurs at the beginning of the next calendar year. In Korea, that's January 1st. In China, the new year starts on February 4th or 5th.
As each new year rolls in, everyone gets one year older as a collective.
So in extreme cases like a late December birthday, you're considered two years old in Korea by the time January swings round. By comparison, you're still a newborn by Western standards.
Here's a November birthday to illustrate the difference:
How to Calculate Your Korean Age
Count one year of age for each calendar year you've been witness to, even if only for a few days.
So if you were born in 1991, you're one that year. In 1992, you're two. By 2016, you're 26. The significance is placed on the communal perspective (the new year) rather than the personal perspective (your date of birth).
In Korea, this is the official method of ageing. It was formally abandoned in Japan, China and Vietnam a while ago, however it's still often used informally, especially among the elderly and in rural areas.
So depending on where you live in the world, you could describe yourself as being 1-2 years older (or younger) than you previously thought.
This got me thinking. Which cultural ageing system is a more accurate reflection of your true biological age?
Your Biological Age
Most people agree that you're technically alive while inside your mother's womb. But there is still some debate over exactly when life begins because it's a Ship of Theseus situation, with no definitive moment to which you can point.
Let's say, for the sake of this particular argument, it's the moment your heart begins to beat. That would add another eight months to your age.
But can you go back even further than that?
How about the sperm and egg cells that fused to form your first complete set of unique DNA? How old are they?
Think about your conception. Not too graphically, mind.
Men produce sperm every day; about 1,500 every second. It then takes 74 days for them to mature into little swimmers with the ability to fertilise an egg.
So the winning sperm that created you adds another 2.5 months to your age at conception, during which time, half of you existed primarily inside your father's testicles.
So between womb-time (9.5 months) and sperm-in-development time (2.5 months), you can add a full year to your biological age. Just how Koreans do.
How many extra months or years can you add by examining your mother's eggs?
For a long time, scientists thought that females were born with a finite set of eggs in her ovaries. Some 7 million, to be not-very-precise.
(They die off inside the ovaries continually, so by the time a girl reaches puberty she has only 700,000 eggs remaining.)
So the egg from your conception was produced inside your mother's ovaries, when she herself was a foetus inside your grandmother's womb.
This leap of yolky logic adds 16-40 odd years to your biological age, depending on how old your mother was when you were born.
However, this may be a bit of a tease. Recent research has identified stem cells in the ovaries that may form new eggs throughout the reproductive years.
But You Aren't An Egg
Good point. You're not an egg—nor a sperm. Those cells are long gone.
So perhaps, in biological terms, you're only as old as the oldest cell in your body right now. Hold old is that?
You might have heard the popular claim that all of the cells in your body are replaced every 7-10 years. This would make you, at most, just 10 years old.
This has a hint of science to it, but is mostly a misconception. In fact, different cells regenerate at different rates, and some don't regenerate at all.
Cells in the blood, small intestine, stomach, cervix and lungs are very short-lived indeed. A matter of days, in fact. This gives them something extraordinary in common with the Indonesian corpse flower which blooms for only 36 hours between years of vegetative growth.
Blood cells and dragonflies also share similar lifespans:
And there's a similar comparison to be made between fat cells and domestic rabbits:
And while neural stem cells in the hippocampus continue to generate new neurons throughout life, some brain cells are simply never replaced.
When it comes to the billions of other nerve cells in your brain, you're stuck with what you're born with.
So this whole idea of cell regeneration does nothing for taking any years off of your biological age. Which leads me to my final point.
Your Age On Other Worlds
Call me frivolous, but I decided to calculate how old I'd be if I lived on the other planets in our solar system.
After all, a year is defined by how long a planet takes to revolve around its sun. And I'm not going to think about the maths any more than that.
So if I lived on Jupiter, I wouldn't be counting Earth years. That would be silly. I'd be counting Jupiter years, where 1 Jovian year equals 11.86 Earth years. Being 33 of your puny Earth years makes me various ages on other planets:
In fact, in the seemingly unlikely event that I ever live on Pluto, at my next birthday I will turn one year old. The date will be Saturday 4th February, 2232, about 216 Earth years from now.