When someone asks in genuine curiosity, I simply explain it, as if it's the most natural thing in the world.

When someone wants to make fun, I just ask them, "How old do you think I am?" Invariably, the answer is 7-10 years too young. Then I tell them my age, some of the things I do on a regular basis, and that I'll be doing this when I'm 90. Have yet to hear any more from any of them.