I think it's mix bag. In many instances it starts with the "baby boomers" who vowed to "never get old".
Unlike previous generations who embraced getting older and relaxing many "baby boomers" strive to look and (act) young. Some have been known to go out clubbing with their adult children.
This was also the first generation that insisted children call them by their (first name) as opposed to Mr. or Ms. That made them feel old.
Baby boomers were the "protest generation", the hippies, the counter drug culture, "free love" whose motto: "If it feels good do it!" This was also the generation that made efforts to be the opposite of their parents! They wanted to be "friends" to their children more than being "authoritative".
Older women today wearing bikinis, mini dresses, and investing money on various surgical procedures to appear younger. More men are getting hair plugs, eyebrow lifts, and trying get a 6 pack in their 40s and 50s. Older generations seek to compete with young people.
Mick Jagger is 72 and is still playing rock & roll music and touring. When I was a kid it was unimaginable to think of a shirtless 72 year old man in skintight leather pants singing and prancing on the stage.
The line between older and younger generations has gradually faded.
Looking and feeling young means being open to doing immature things. Conforming has always been seen as being a "fuddy duddy"
Today we have Jennifer Lopez at age (46) who recently had a hit called "Booty" with Iggy Azalea and even Madonna at age (57) still attempt to use sex appeal to draw an audience.
Our insistence of "staying young forever" oftentimes leads us to displaying immature an adolescent behavior from time to time.
There's one caveat though. We're much more "politically correct" today when it comes to using words and phrases for descriptions.