If people ever ask me how old I am I usually can't remember. It keeps changing.
I do know which year I was born, so I have to remember which year it is now (which also keeps changing), do a bit of mental arithmetic, then take into account the time of year (it keeps changing as well) in order to work out whether I have had a birthday yet.
With luck, I will come up with somewhere near the right answer.

I knew my mother was getting old when she said "I'm eighty-three, you know."