Of course 30-somethings can like robot godzilla movies, but I think the force with which reddit upvotes stuff like Avengers and that Godzilla movie is an indication that the demographics have shifted to high-schoolers and early college instead of college and older.

Maybe you're right about subreddits, but I still think it'll hurt the subcommunities to be associated with a larger more immature whole. Now if reddit was just search engine for forums and each forum was more independent, that would be different.