Hmm... I'm fifteen and have just recently found the site. I really love it, but I only share certian episodes with my sisters. It really isn't a site specifically designed for children, but it's nowhere near as bad as South Park, the Simpsons, or Family Guy. The creators specifically try to avoid swearing and vulgarity. However, the characters drink alchohol, occasionally smoke (although the only main character who actually is a smoker is The Cheat, and they rarely show him smoking), and there are a select few scenes in certian videos that are a little iffy.
The site does also show blood on occasion, and does have toilet humor in it, although it doesn't really run on that type of humor. It's mostly just full of in-jokes that you slowly understand more and more of the more episodes you watch, quirky humor, and references to 70s, 80s, and 90s pop culture, video games, and music.
Most of the time though, I can show these to my younger sisters and we all enjoy them together. They're REALLY funny, and I've often laughed out loud at them (something uncommon for me even with cartoons that I really enjoy). Just note that the characters are certianly not role models of any sort, so you might want to make sure that your kids know not to follow in Strong Bad's footsteps (although it's pretty obvious that he really doesn't ever know what he's talking about).
I don't see why it got rated iffy 15-17 though. Maybe if you're hyper protective/protected, like me... I don't think I could have brought myself to watch this a year ago; the references to alchohol would have creeped me out. But South Park is ridiculously worse. I'd reccommend maybe, say, 10 and up, although it really varies between toons.
Sorry for the long, mixed up review.