The thing is, it's not the kid, it's the kid's brain. And although by age 9 or 10 that brain is the same size as yours, the self-regulatory centers -- the decision zones that govern judgment and impulsivity and should scream, "Hello?! You could break both your legs jumping off a roof!" -- are the last to reach maturity. That doesn't happen until age 24 (later for men). Result? The adolescent brain has adult-size pleasure and sensation centers without adult-level control. Explains a lot, doesn't it?
So how do you keep your kids safe until age 24? Saying, "What were you thinking?" afterward doesn't work. Instead, you need to get them to practice a bit of delayed gratification. As in: You'll have more fun (be more popular, win more games, get along with the principal) if you don't go for the immediate gain (for example, being the center of attention for 5 seconds before breaking both legs).