Charles M. Schulz is my all time favorite cartoonist. He is known for creating PEANUTS. Adults never showed their faces in Charlie Brown's neighborhood. You knew they were there, they just were never drawn. As a kid who loved reading every Peanut's strip that came my way - I always wondered what a grown-up looked like in Schulz's world.
A friend of mine, Brad Condray, once gave me a book of cartoons with adult characters called IT'S ONLY A GAME that featured adult characters. That's right, Sparky had a sports themed strip that had a short run from '57-'59. He also did a single-panel cartoon panel called YOUNG PILLARS from '56 to '65 for a youth magazine called YOUTH. There are two examples from each series.
What's neat about seeing these works is that you can make out teenage and grown-up versions of Charlie Brown, Linus, etc. I don't know if it was intentional - but it's easy enough to pick up on.