Could you clarify regarding the comment quoted above? Are you referring to your school or system, most MA schools, or are you describing what you feel to be the situation in general with martial arts schools today?
I ask because the system I train has been largely focused on teaching adults. That was also my personal focus running a very small school, ...and I recently closed my doors. Even our association HQ has diminished in size and the chief instructor now works a second job. On the other hand, it seems that some MMA and jujitsu schools seem to do just fine focussing on mature teens and adults.
I was referring to most of the traditional martial arts schools in my area, including my own school. There just aren't enough adult students to keep the doors open. FWIW, I don't think it is all that different for BJJ schools, but frankly I don't know for sure.
IMO, there are a couple of reasons for this. Adult students have many demands on their time including work, school, children, and spouses, not to mention lots of options for both social and athletic activities. Unlike other recreational activities, martial arts demands a high level of commitment, mental energy, and most of all, consistency. You can enjoy, say, golf, tennis, or skiing, and stop for long periods of time, then take it up months or even years later. Alternately, if all you need is to get some exercise, you can find activities you can do whenever, such as jogging on a treadmill, stationary cycling, or weight lifting that you can do whenever. Martial arts means getting to a Dojo/Dojang on a certain time on a certain day, which isn't always possible for some people.
Here are some real world examples of adults/older teens I knew from my school who quit this year.
1. A guy who made probationary black belt. In some ways, he was one of my role models coming back as an adult as he was already a high level intermediate when I was a white belt. And like me, he was a bigger guy and also middle aged. And he made probationary black belt, then had to take some time off because of a back injury. Then he just never came back. Apparently, his son, who also made probationary black belt decided to do football in the fall and so dropped Tae Kwon Do. So this guy felt that while he might have the time to do TKD with his son, decided he , didn't have the time train TKD and help with his son's football practices and other activities.
2. A guy who, like me was middle aged, and was a few belt ranks ahead of me at the beginning, but was lagging behind because his work schedule didn't give him enough time to train. He told me he intended to make more time for TKD, but then a month or so later, he abruptly quit because he said he just couldn't spare the time. I recently saw him and he is thinking about coming back, which I hope he does.
3. A woman who made 1st Dan, who used to train with her teenage son who also made 1st Dan. Her son quit, and so after a time she decided to quit, though she told me she may come back next year.
4. An older teen who had all the tools to make black belt, but who just became more interested in acting, and so didn't have the time to continue.
5. A woman who also has all the tools to advance in TKD. She is tall, athletic and smart. But she told me her work schedule was getting hectic and her kids had a lot of activities she needed to help out with, so she is technically on a "break".