Adults envy those younger than them because they remember the "good-old days" when they were young and had not a care in the world. Why, then is the adult population so intolerant of the actions preformed by these youths that are the focus of such envy?
Adults have something that no youths have; age. With age comes experience. Experience usually means that the adult has some sort of completed education and has an understanding of the laws and the way things work and operate. They are not used to things operating differently than they did back when they were young, thus resisting change and much of what the young represent.
An adult can look at some youths fooling around and think that it is some all-time low, or something that that adult is above doing. The adult is forgetting what it is like not having the point of view or insight of an adult, and is choosing to ignore the obvious entertainment that it would have brought to that adult back in the "rebel-years" or the "good-old-days."
Sometimes, the adult will remember the entertainment that certain teenage activities brought to him or her only a few years ago. The adult will have dozens of excuses for not performing the action themselves; "Oh, that's for kids;" "That's illegal;" "I know better..." "It's different now."
Perhaps the most envied portion of "youth" that adults envy is the literal word, youth. Adults want to be younger, in order to live longer. Youths often think of themselves as immortal, or, more appropriately, adults often think that youths consider themselves immortal. Young people can not care about anything happening anywhere in the world, and still have fun while ignoring an event happening just a few blocks away from home.
Adults are faced with worries. Youths are not. They don't (usually) need jobs to support families or just themselves, they don't need to worry about jury duty, taxes, inspections, exercise, or their weight. They don't need to look out for elderly parents or young adventurous children. They seem to have everything that adults lack.
There was a Family Circus cartoon in the Boston Globe a few weeks ago that had one of the children relaxing on the roof, telling a sibling that when she grew up, she would do anything she wanted. In the background, there were children all over the place, playing in the streets, climbing ladders, playing basketball, and other activities while the adults were mowing the lawn, trimming the bushes, taking out the trash, going over paperwork, repairing the roof (and trying to work around the kids on top), and cleaning the toys from the floor indoors. This is a perfect example of the viewpoints of both sides of the coin. Both sides feel restricted, both wanting to be in the others' place, yet both would realize that they would only strive to change such a swap back as soon as it took place.