As a comics fan who isn't quite middle aged yet, I can say that for me it's a combination of things. For one thing, I've read them on and off since I was a kid, and I enjoy the entire format. In addition to that, the medium as a whole has evolved now to where many of them are written for an adult audience. Book's like Grant Morrison's Batman R.I.P. or Alan Moore's run on Supreme have deconstructed and reassembled the superhero genre to the point where some books can be very challenging on more than a few levels to read. There are more than a few books that are superhero books that I still read, if only because I find the story interesting and suspenseful. Greg Rucka, Fraction, Brubaker, Morrison, Vaughn... these guys all have CHOPS. It should be no surprise that lots of these guys are getting tapped to write for other media, nor should it be a surprise that a lot of them choose to keep one foot in comics. Not to mention that the artwork in most of their books is incredible.
There are still more simplistic superhero books that I read, too. Kirkman's Invincible is a good example, or Bendis' New Avengers are good examples. I don't think the storylines are anything complex or highly developed, but both writers have a great sense of storytelling and and a penchant for dialogue.
Also, a lot of it might come from my being a huge nerd. Either way, they seem to be more and more a part of the collective pop culture. Unlike the bulk of our pop culture, there are comic books that can be decidedly high brow.
or, I guess to put it a different way, can someone explain to me the appeal of Two and a Half Men to middle-aged men?
re: Grant Morrison
I really liked The Invisibles, but it definitely takes some getting used to. Peyote might help. For some reason, I tried to read Filth first and it really confused the hell out of me. As far as his other works, I'd recommend his Doom Patrol stuff, All-Star Superman, or We3.