That's where Spike & Mike's Sick & Twisted Festival of Animation came in.
You'd go to the movie theatre and watch a shorts series, probably 20 or so animations, that were 100% for adults, though regularly attended by teens. The series had some stars - While not the true launching point for South Park and the shorts of Bill Plympton, this was certainly where they both became super-stars on their way to greater stardom.
IN 1995 or so, I went to Spike & Mike's like 10 times, and one of my favorite films was TOny Natoli's I Never Ho'd For My Father, in which Santa Claus is more or less a dirtbag thug, nailing the wives of his elves, doin' lines of coke, and slappin' folks around! He's awesome in a Joe Peschi sort of way. To me, this is the vision of what Spike & Mike's meant. The animation isn't great, it's not polished nor fluid, and th esound is weak, but it makes you laugh, hard. It's over-the-top, totally over-the-top, but it works so damn well.
The fact is today it's harder for these kinds of film to find that audience, and at the same time, it's easier for 'em to find viewers. YouTube and Vimeo are full of these kinds of animations for adults and they're getting tons of hits, but when you've got Archer and Adult Swim, you've not missing out on anything in the mode of mature animation. SPike & Mike's is still around, from what I understand, but it's no longer the stalwert.