Like everyone else mine changes every time I go on there. The youngest I have been is 22, and the oldest is 35. I'll be 26 next month. :) It generally averages about 29, though. Guess I'm a bit out of shape still.
My wii fit age is changing almost daily, depending really on how tired I am, or when I do the test, but actually it is tending to be more like the real age, I'm 36 now and my wii fit age is from 60-28.