I feel I don't really encounter the same bliss people have for youth in modern times when I read older literature. From what I read in psychology people who were miserable their entire life but had a nice time last week are really nostalgic about last week.
The way I see it our present culture came into being during the 1960s, that's when a generation much larger than the previous ones came into adulthood. The 1950s are pretty irrelevant in comparison. But back then being a youth really was bliss, what seems to define that time the most seemed to be collectively dropping out of society, due to a collective skepticism about what was demanding any kind of sacrifice or responsibility from them and so many of them honestly believed their promiscuity and drug use weren't selfish but that it instead was going to bring about world peace and save the world. The sexual armsrace was not developed, supermodels from that time look downright decrepit by many people's modern standards. Hairy armpits, they seem so relaxed about everything in comparision. All the joy of hedonism with no consequences yet. Junkies had not begun littering the streets of every city the way they do today.
Exiting youth, growing up surely also means coming down in a big way for people who lived that kind of life, from way up in space or cloud 9. You could see how a lot of them would see youth as an unimaginable happiness and adulthood, where they have to live in the world they created, an unimaginable suffering. The later youth culture in the 90s seem endemically depressed. Pop/rock idols didn't overdose on drugs or crash in planes or expensive cars anymore, they just killed themselves.
Interesting take on youth.