I think age is a factor, and I think that's natural. Fer'instance, my interest in metal waned about the time I turned 22 or 23. Death metal, grindcore, black metal, nu metal.... it doesn't really do anything for me, but I also think it's not being made for me. I'm just not that pissed off anymore.... cranky, maybe, but not all that pissed off all the time. I don't worry too much about it, though. I have a ton of music, new and old, that I love a lot. I give new stuff a listen, and if I like it, it becomes part of the legion of stuff I listen to. If it doesn't do anything for me, I don't sweat it too much or worry about the generational divide. I just move on to the next thing I haven't checked out yet.
For the record, I just turned 39 about 3 weeks ago, so in this day and age, I'm certainly not "OLD", but that is definitely middle age knockin' at the door if my expected lifespan is in the 78 year range. But that's bullshit as far as my ability to discover or enjoy music goes. I hope I'm still curious about new shit when I'm 78... in fact, I'm gonna make a point of being curious about it.