In my experience, although the term is used a lot in this sense, "maturity" has very little to do with it. If anything, people who say they've "grown out" of an entire genre, band, etc. probably just associate that music with a time in their life when they themselves were less mature.
The bottom line is, at least for me, the REALLY good music doesn't ever get stale, regardless of genre. I may tire of listening to it for awhile, but I still appreciate its awesomeness. For example, when I first discovered DT, I devoured every song of theirs I could get my hands on, and liked practically all of them to a degree. Nowadays, a fair portion of their work does not interest me, but it's not because I was immature for enjoying it previously. Their best music simply withstood the test of time while some of the lesser stuff did not.
As for the genre thing, I enjoy a pretty wide variety of different music, and I could not say that one genre is more consistent than another. Every genre has its share of garbage. With progressive rock it is a bit different, only because the genre is so poorly defined, and it often falls victim to things like extreme self-indulgence and pretension, which we as prog fans don't mind if done with a bit of subtlety and taste, but it's a very fine line. Where the fuck are rob and his masterful command of English to word this eloquently for me?
-J