It's not, I'm 30 and I still don't feel like an adult adult.
Part of becoming an "adult" is the realization that "adulthood" is a mindset and not something you arbitrarily become as you get older. Once you realize that people in their 50s can throw temper tantrums at work or elsewhere, the belief that older people know what's going on and have it all figured out starts to crumble. There are also plenty of 40, 50, and 60 somethings with the maturity of somebody in their early 20s.
In addition, being an "adult" is also the outward projection of normal and responsible behavior. It's a willingness and the action of putting aside the things you want to do for the things that you should and have to do.
Some people are just really good at constantly projecting that outward image and keeping that fixed mindset of responsibility.
You lose touch because our societal constructs are built to keep you occupied with different time-demanding activities as you grow older. In our thirties, we've got our full attention on a very narrow, very specific thing (or things) -- work, and relationships. Playtime is relegated to what we took with us from our childhood and teenage years; what were most comfortable with. And it usually means we grow out of touch with the next generation.
You'd have to actively stay on top of shit in order to feel like time has never left you, and that can get tiresome. You also risk success in the other things that have pushed upwards towards obligational priority in your life.
I always chalked it up to a few things.
First, it's the belief that what we had growing up was mostly unique and special. As an example I'm 38 so it's going to be stuff like the Ghostbusters, Star Wars (even though this was actually before my time in most ways), He-Man, Thundercats, TMNT, the NES/SNES, Nick Toons, etc. As a teen, it's going to be all kinds of 80s and 90s music with different musical movements like gangster rap, nu metal, grunge, alternative rock, and on and on.
Second, the novelty of new things wears off and as you get older you start to see the commercialization motivations a lot more behind many products.
Third, it's my understanding that physically our brains aren't as receptive to things like new music as they were when we were teenagers. Or it might be our emotional ages where we don't relate to the words and emotions of the music as much but we can with older music because we already built those attachments.
Fourth, and this is basically what you're saying, it's that we don't have the time or even the interest in staying in touch with what's new. Even with lots of free time, there's more media than ever out there even if you just wanted to cling to those past IPs because many of those properties are still getting new content. While working, it takes me a solid month to get through lots of video games or hell, while not working it takes me a couple weeks still. There's never been as much media content as there is today and it's just exhausting, like you said, to keep up with the past much less with what's new.
Lastly, I think for many people their interests change from enjoying things that have a short product cycle (the latest band or whatever) to things that are more universal like spending time with family, outdoor activities, home projects, or skill based active hobbies like learning musical instruments, knitting, car maintenance, etc.
Again to mirror my previous post, aging really isn't all that bad nor is losing touch with what's "in". If 30 is old then we spend 50-66% of our lives as "old".
