My son had a birthday recently, and has become kind of obsessed with how old everyone is (he's also taken to calling his dad "old" which isn't winning him a lot of friends).
Which has me thinking - how old am I really? Sure, the calendar says I'm 36, but what about at a cellular level?
Even though we might not realise it, we're constantly growing and changing. Every day some of the 30 trillion odd cells that make up our bodies are dying, and are replaced as other cells divide to create more.
The process of cells dividing is called mitosis.
This constant cell turnover means that some parts of us are, in effect, younger than others.
Each different type of cell has its own calendar when it comes to aging.
Sperm cells are among the shortest-lived, with a lifespan of only a few days.
Some cells, like the cells lining our stomach, intestine and colon, have a pretty rough life. They only live for around four or five days before being replaced.
Our skin also takes quite a battering. The cells making up the outer layer live a bit longer than our digestive cells, maybe lasting for between two and four weeks.
Your red blood cells (unique because they're the only cells that don't have DNA) get replaced after about four months of circulation. Some of our white blood cells (immune cells) can live for years - which helps with long-term immunological memory.
Although these cells can divide, there are some limits - they can't continue to do this indefinitely. Something called the Hayflick limit describes the number of times a cell can undergo mitosis. Each time a cell divides its telomeres (the ends of chromosomes) get a bit shorter. Once telomeres shorten to a critical length a cell will no longer be able to divide.
As our cells age and reach these limits, so eventually do our organs and bodies.
Not every cell can divide - there are some cells that we're stuck with for life.
For example, females are born with a certain number of oocytes (immature egg cells), and that's it. Those cells don't get replaced as we age.
The majority of our neural cells also don't divide, and aren't replaced when they die.
But if you look at it at a cellular level, despite what my son says (and how I feel some days at the moment), I'm not really that old after all.