What's my age again?

I turned 32 last month and have to admit it’s becoming harder to deny I’m getting old. The emphasis on early in “early thirties” is reaching its limit and I think it’s time to reassess what my age actually means in an attempt to sound younger.

So let’s start with the basics – a year is the time it takes for the Earth to orbit the Sun. Since I was born the Earth has travelled around the sun 32 times. It has a nice cyclical feel to it and is a sensible unit of time, at least for us Earthlings. If I’d been born on another planet my age would have been significantly different. Each planet in our solar system takes a different amount of time to orbit the Sun meaning they have different “years” to us. Have a go for yourself here. If I’d been born on Mercury, which orbits the sun every 88 days, I’d be a stately 133 “years” old and if I moved further out into the Solar System and had been born on Jupiter I’d be almost 3 “years” old as it takes 12 times longer than Earth to orbit the Sun. There is a drawback to this younger age though as the gravitational pull of Jupiter is 2.4 times greater than Earth’s, meaning I’d weigh a lot more on Jupiter than I do here. If I take orbital period and gravity into account then my ideal home would be Mars as I’d only be 17 “years” old and would have lost two fifths of my weight. Maybe I should volunteer for the eventual Mars expedition?

Even if I could reach these planets I wouldn’t be able to live on them and I’d still be 32 on Earth so I need to find some other definitions of age.

As a living organism there must be a biological context for determining age? Our hearts beat throughout our lives and stop when we die so this seems like a fair measurement. My current heart rate is 65 beats per minute (bpm) so if I assume that’s the average speed throughout my life that would bring me in at 1,095,494,400 (let’s say a billion) heart beats. There is a worrying catch with this number due to a rule-of-thumb that predicts animals only live for a billion heartbeats  (derived from Kleiber's law and observations by Raymond Paul) . This isn’t literally true but is based upon the observation that smaller mammals have faster heartbeats and shorter life spans than larger ones. A mouse has a heart rate of 500 bpm and can live for up to 3 years while an elephant has a heart rate of 28 bpm and can live for 80 years. When applied to humans this would mean we live for around 33-35 years which actually matches some unfortunate parts of the world and was in line with that of our prehistoric ancestors (although it's virtually impossible to know what lifespan is in "control" conditions).  It's mainly advances in healthcare and hygiene that allow us to last considerably longer than projected. Things like an active and healthy lifestyle also help us live longer, so don't avoid exercise to conserve heartbeats!

Considering heartbeat ageing would have resulted in my premature death, I’m going to avoid using heart beats as a measure of age as it makes me seem even older.

How about the age of my body? It's not actually as old as you’d think, as cells are constantly being replaced throughout our bodies due to wear and tear and the Hayflick limit dictates that the average human cell can only divide 40-50 times before it becomes senescent and can no longer divide. Different tissues have different rates of replacing cells meaning some parts of my body are much younger than other parts. Our skin is constantly being damaged by the outside world and, on average, is never more than 35 days old* as that's how long a skin cell lives for. My red blood cells are never more than 100-120 days old and even my skeleton is replaced over the course of 10 years. The only parts of me that are 32 years old  are non-dividing cells, such as the neurons in the cerebral cortex of my brain, the majority of my heart muscle cells and the cells that make up the inner portion of my eye lens. Technically they are “older” than me as they appeared while I was developing in my mother’s womb. On average it seems the human body replaces itself roughly every 7 years which means while I am getting older some parts of me remain quite young. Like a Time Lord from “Doctor Who” you could say I have regenerated four times since birth J

In conclusion I think it’s safe to say that age is very subjective and can vary depending on how we choose to measure it. Maybe we really are as young as we feel or not as old as we fear?

This song seems apt.

* The lifespan of various cell types vary depending on the technique used. There is an interesting variety used in research and the numbers I've used are either averages or those taken from a Carbon 14 method used by Dr Frisen. as that gives ages for tissues as opposed to cells. So don't be alarmed if there are any discrepancies.