I'm a jazz musician, and my kids are both professional classical players. I've asked them why they don't learn to play jazz. My daughter described pretty much what Glass is saying here. She calls it "fear of sucking." She knows what good jazz improvisation sounds like, and trying to make herself do it is pretty discouraging.

Not that there's anything wrong with loving and playing classical music, which is a factor too.

This may be why it's different when you start very young. You're not conscious of your own sucking, you just play, usually in a setting where everybody's congratulating you. For sucking. ;-)

I started on classical, and got into jazz by accident, as a bassist. It turns out that you can function in a band as a bassist without having to improvise very much, so I was able to learn at my own pace and eventually did. In fact a lot of good jazz players started out in school jazz bands or large ensembles where you didn't have to be a good improviser right up front.