Judith Berman has an excellent article on her site about the aging of SF readership. I've corresponded with her about this as well. I think you've hit the nail on the head when you talk about the cost barrier for younger readers.

When I was a kid, (early 70s) I could buy a bottle of pop and some candy and a book or two with my allowance which was just a few dollars then.

A candy bar or soda cost about the same as a book or comic book then. Today a bottle of soda or candy bar at 7-11 costs about a dollar. Books cost $7. Comics $3 or more.

I think this has a great deal to do with the aging of SF readers. Basic economics.

Ms. Berman points out another economic problem with SF is in the short story market. The rates paid there haven't changed in, what, six decades?

Kermit