A lot of model railroaders picked up the hobby as a kid, then drop it when they reach girl-chasing age. As age catches up and spouses actively deter girl-chasing, hobbies like model railroading reappear. This was my pattern: active model railroader as a youth, still interested as a teen but it took a backseat to other pursuits and was nearly forgotten in college and my twenties. In my early thirties I was married and bought my first house, and suddenly realized I had enough time, space and disposable income to get back into model railroading.
I'm 39, definitely in the "Generation X" age demographic, and retirement definitely does mean something different to people my age: it's something we plan on doing, but don't necessarily want to wait until we're old and gray before we start enjoying our free time. Which is why I figured it was time to start my layout now, instead of waiting until retirement.
Yes, people are more into instant rewards--but there is a lot more RTR stuff on the shelf, including structures, and while the Internet has made life a bit harder for bricks & mortar hobby shops, it also allows suppliers to reach a worldwide market and allows those with shared interests to meet people they would otherwise never contact. On this forum we can encourage people half a world away to learn more about model railroading and teach them about the hobby.
There will always be a few people in their twenties in the hobby--but they're the exception. This hobby is for kids, I just happen to know a lot of balding and gray-haired kids. People in their twenties want to act grown up, but some of them will rediscover trains when they're old enough to start playing again.