Hard Headed Teenagers

Why is it that when we're teenagers we think everything that older people tell us is a crock? We all at one point or another heard some older people talkin about how if they "knew then what they know now things would've been so different" or "when you're older you'll understand". Well I find myself being that old person (at least in my kids eyes)and its kindof sickening. When they say things like "It's not like it was way back when you were a kid" I wanna just knock some sense into them, but you can't do that without getting CPS called on you. Were we really that dumb when we were their age? I'm torn, should I keep trying to teach them what I know about life or should I just let them make their own mistakes and hopefully learn from them?