I think part of it is that when you hit a certain age, as music obsessives, you have a METRIC $#!% TON of music-listening under your belt.
It gets to the point where it's like "Well, I've heard this before, and I liked it more when it was more original."
I often get turned on to a band and think "Well, this is okay, but they are just ripping off (insert name of a band that was doing something similar earlier on)."
And that's okay. Originality is not the be-all and end-all of making or listening to music. If it were, we'd have precious few things to listen to!
But with lyrics in particular, whatever genre of rock or pop or whatever, I just feel like I've heard it all before, heard it done better, and there's nothing much I can get out of listening to a 20 year old expel his or her angst or other emotions through their "heavy, hardcore lyrics, maaaan."
I just kind of roll my eyes most times.
I do still dig on listening to older stuff I always liked that has vocals...that's just nostalgia.
I also find that a lot of the loudfastrules punk stuff I listened to all through the 80s doesn't hold up for me very well, and it's mainly the material that has overtly political lyric content. Unless there is some humor mixed in with it (Dead Kennedys and Crucif**** come to mind) I just can't hack it much, even though there are still loads of punk from that period I still spin now and then.