Why do guys tell their friends about their sex life?

At that age they feel like they are better than their friends when they do that, so, they do it without consideration for the girl.
At that age guys feel the need to compete with each other, even when they are friends.

Yes, they're trying to sound experienced. It's a sign of immaturity that they can't keep personal matters to themselves.

Its because some guys want to feel accepted and they want to grab attention and chances are none of their friends are never going to really find out if they are telling the truth like if I said I built the best tree house they are going to want to see it and if they don't see it they will see that your lying but during sex all they have to believe is your word and there is no proof of it actually happening so that's the only way we can make ourselves look better in front of other guys without the chances of being proven wrong or made less

I found when I was younger my friends that bragged about getting it all the time. Were not really getting it. Friends that did not brag or really talk about it were actually quit busy.

Guy's and young guy's in particular seem to feel the need to brag about their sexual adventures. Personally I was always much more discrete (or maybe I just didn't have anything to brag about). But then girls aren't exactly shy about telling their friends about their love lives either

To brag. That's my vote!

Ok so guys talk about their sex lives so what girls talk too. And if you want to get right down to it ,a guy may say tell his buddy yeah I banged her, but a girl will tell her Girlfriend so this is what he said and his hands were right here and he held me like this and looked at me like that. So let the guys talk it's normal, women talk too, they just give more details then most guys do.