Drinking was never overhyped for me, I had my first stiff drink at age 12. Watching other people get excited over it has been amusing, though.
Same goes for driving... I learned to drive in a field in my uncle's '66 VW in my early teens (had to do it barefoot, thanks to not having pedals. I actually learned to grip the clutch, gas and brake levers with my toes. To this day I unconsciously curl my toes when driving). It's mostly just a responsibility not to get yourself and your passengers killed.
I knew exactly what to expect from high school, thanks to my older brother... and it sucked.
Sex... that was over-hyped. My first time was drunken, awkward, smelly and generally awful. Don't let that put you off though, kids... it's awesome if both parties know the basics of what they're doing and clean themselves properly... but to be honest, I've had some really bad sex in my time, and masturbation is far better than bad sex.
Killing other human beings... still not sure it was me who made the kill, the first time. At range, it's mostly just confusing. Never killed a man close enough to see his eyes, and I hope I never have to... which is an odd dichotomy, since I'm always armed and prepared to.