A couple key points people are missing:
1. This hobby isn't cheap. If someone is looking to get into the hobby a decent starter set is around $200. Not a whole lot of people have hundreds of dollars lying around leftover from their budget to spend on expensive hobbies, especially people in their teen years, early 20's, or those just starting a family. In high school, undergrad, and my master's program I was out of the hobby as I didn't have the extra money or time to spend on it. Several years out of school I now do.
2. Space. If one doesn't have a house with a basement it's hard to find the space to build a layout. Many younger people live in apartments, possibly with roommates. They may not have the space for a layout.
These are probably 2 of the biggest factors in limiting entrance into the hobby. Think back to when you were young. It would have been pretty hard to justify dropping $1500 on a locomotive. As people advance through their careers (and age) they end up earning more and having some disposable income, hence why there are more older people in an expensive hobby.
