15 years old, and I get depressed when I know a lot of 15 and 16 year olds, all older than me, who are extremely dumb and/or immature, both online and offline.
And really, the fact that people aren't usually lying, isn't really true.
I'm sure there are just as many people who do lie about their age (Young kids who don't want people to know they're so young, generally for safety).
It's probably near impossible to tell though, as on the internet, you almost never get to see behind the mask of a persons computer screen.
Of course there are those who say that they are their exact age, even if they don't fit the stereotype of people their age, but those stereotypes are established for a reason. They quite often contain some truth.
I'll still take peoples ages with a pinch of salt.