How old are you?

It's amazing how people will lie about their age for apparently no reason.
I'm talking to a young man from South Africa, he tells me he's 18 -oh! a young one!, I thought- then, he invites me to view his web cam: he looked like a kid. So I tell him, ''you look young'', more like 16, and he admits being only 15 (!!!).
I'm wondering just how many people lied to me about their age (?). I usually give people the benefit of the doubt, I accept what they say but I don't put faith in it, it's not like they can actually prove any of what they say about themselves!