Check your birth certificate
it's today's date minus the date on that
It's the number of years you've been alive
It's the number written on the front of your birthday cards - if they've sent you any
what does your wife say you are?
Maybe it's something to do with you shouting at the adverts on the telly
it's how knackered you feel having just climbed the stairs
or the way you groan after getting out of your chair
it's how you feel after seeing an old school mate
it's as old as not remembering why you went into the kitchen
as old as how long it takes you to 'wee' at 2am in the morning!
Maybe it's your shoe size?
I think I'm 63 - but with a mental age of 12!