Life lessons.
Good judgement comes from experience, and experience comes from bad judgement.
Elders generally make good decisions because they have lots of experience from making lots of bad decisions. Listening to them can help keep you from making the same bad decisions they did, and instead make your own unique bad decisions. Thats how we make human progress.
So next time some one who is older trys to tell you not to do something, realize that it might be because they allready tried that, and it didn't work out to well.
Also there is a difference between being stupid and making bad decisions, making bad decisions is part of growing up, and usually involves lessons being reinforced with the natural occurrence of blunt force trauma (one of my favorite motivators in the world). Being stupid is thinking that the Occupy Wall Street movement had marett. I personally think it is awesome how it is imploding on itself, and the smarter ones among them (that's like being the smartest kid in the special olympics) are realizing how fucking stupid their ideas were. Reallity is starting to hit those camps pretty darn hard. /tangent