This is also because laughing at jokes is a way to (unconsciously) show that you get it, and/or belong with the "in-crowd". So people tend to laugh at jokes other's find funny, even if they wouldn't themselves, and remember the joke must've been funny because they laughed. Also see: laugh tracks.
I know what you mean, but it isn't quite the same I think. The anecdote I told in a sibling comment about a friend and me sitting in the back of my mom's car is an example. We were two very good friends, so there was no "in-crowd" dynamic at all, and we were absolutely loosing over it, it was genuinely funny to us. Roughly the same age as what OP stated, so there just seems to be something inherently funny about those non-sequiturs to young folks... maybe that's when you start to "deconstruct" concepts like humor.
