For one, all this epitomizing of youth only emphasizes the upsides and thus avoids the downsides of being young. There are many downsides and arguably only a single upside. Ignorance, immaturity, and naiveness are all downsides. When you are eighteen or even into the twenties, you have very little understand of life and can easily succumb to social pressures. I think we have all read stories (or even personally experienced) feelings of inadequacy, and misery to the point of harboring suicidal thoughts. Arguably there is only one upside to being young-- a healthier body with which we can better compete in sports, dance, and travel around. But even this is not completely true. Just think about all the examples of those labeled "old" in their active endeavors: Dana Torres, the 41-year old Olympic swimmer; Michael Jordan in his last comeback tour with the Washington Wizards; and Dick Hoyt, the father who competes in the Iron Man Challenge while carrying his son Rick.
Perhaps what creates this emphasis on youth is not about the joys of being young at all. Instead, it is a reflection of unsatisfied/unfulfilled lives. Lives that have not found meaning or purpose. Because if one has, then one would not want to go back to the way things were. One would want to always moving forward, desiring more -- to do more, see more. It's like this: why would you want to go back to the dark when you have seen the light? Furthermore, the desire to being young again ignores the "enlightenment" that comes from the wisdom derived from life experiences. Returning to when we were eighteen would mean losing out on all the friendships, life events, and knowledge (aka experience) accumulated since that period of our lives. I'd consider that to be a very unequal trade...for the worse.
One book I read that profoundly discussed this preoccupation with aging is "Tuesdays with Morrie", by Mitch Albom. The book is a recount of the lessons passed down to Mitch by his professor (named Morrie) in psychology at Brandeis University. Morrie discussed a wide-spectrum of issues that confound today's society and singled out this worry about aging in one chapter. In particular, he states that: "It's impossible for the old not to envy the young. But the issue is to accept who you are and revel in that. This is your time in your thirties. I had my time to be in my thirties, and now is my time to be seventy-eight".
His point, which I wholeheartedly agree with, is that aging will happen regardless of whether you like/want it or not --instead, the focus should be on the present and making the most of every opportunities you have. Sometimes it's about taking a chance and leap into the unknown. Because the worse that can absolutely happen is gaining experience.