I was under the impression that the amygdala (that's what you're referring to, right?) wasn't fully developed until (okay, on average) until 18, not the early 20s.
Either way, one's labido gets the better of many people, regardless of age... maturity often doesn't have as much to do with it as one would hope.
But.. umm.. yeah... more sex will cause STDs, that's not news
STDs are up everywhere, though. And it's unprotected sex that causes them (or... not knowing how to use it..), not the sex itself. Many conservative programs are screwing kids over (so to speak) by teaching them only abstinence. That will cause STDs, because, as I always say starting now, there are three certainties in life: Death, taxes, and teenage sex.