Hmmm...this is interesting.
The age generalization is a tricky issue. But I think it is there for a reason. There is no question that as a teenager matures, he learns more, and realizes that he didn't know as much as he thought he did. He also realizes that he wasn't right all those times. The old people already know this, and so when they see smart-ass knowitallism from some punk, they remind that person of their age, and point out that in X amount of years when they own things they might think a little different. Of course, young kids are too hot-headed to realize this, and remain confident that they will continue demanding anarchy when they own a house and have a family.
My issue with age generalization is when it is misplaced. I see a lot of young people who present very good, thoughtful points, and the old people write it off as immaturity. "Oh, you just disagree because you're young". Well, maybe that's true on some issues, but not on everything. In fact, you hear many parents talking about how they learn the most from their kids, especially on how to be humble. Doesn't that seem like a contradiction?
Is some punk kid who wants to destroy the system going to change his ways in 20 years? Yeah, almost definitely. And when you see this punk kid running his mouth, there is nothing wrong with pointing out his age and inexperience.
Is some teenager going to change his opinion on the death penalty in 20 years? Probably not, but maybe. Either way, it's a perfectly fine opinion to have on something at just about any age, and because of that old people should just leave age out of it.
Basically, when old people use age as an argument during a debate where it doesn't belong, that's when it gets on my nerves.