I don't think that jazz was ever a young people's music. When I was coming up, I made a conscious effort to seek out music that had more depth than the rock n roll I heard on the radio. I basically did that on my own. I bought some copies of down beat, used those to learn a few names and pick up some records, and was on my way. A couple of years later, I came to blues in pretty much the same way. Back then, I was the only person in my high school who listened to jazz - at least I didn't know of anyone else. Even in college, real jazz fans were scarce.
Imo, jazz or any other music that requires true listening - as opposed to surface listening - will probably always have a minority audience, and probably a more adult audience. I don't know that that's a good thing or a bad thing. It's just fact.