Whilst 47 is the conventional answer I have another more philosophical one. I am as old as the universe, because the sense of self is possibly a universal experience of all matter. Therefore I am not just the guy in this body but am also all things everywhere. I just cant feel my toes so to speak. Think about it. More awareness of a point like that could lead to world peace and harmony, or maybe just a few more admissions to mental health units?
Others might put different answers:-
The matter I am made up of now is not the matter I was born as - it has all changed around many times. So in that sense I am only a few milliseconds old as atoms are being exchanged with the rest of my environment all the time.
Then again my short term memory seems to be getting shorter, can only remember back a few seconds, so maybe that is how old I am.