The Simpsons is a television cartoon (primarily) for adults and should be classified as such.
Throughout history, The Simpsons has always been geared towards adults as opposed to children. Matt Groening himself had that intention in mind back in the early 90s. The target audience for the show was (since its infancy) ages 18 to 49 years and it successfully won its audience. Seasons 1 to 3 seem kid-friendly now (I agree) but it really wasn't until Season 4 when The Simpsons took the more adult/offensive/explicit end to things which I think is fuckin' awesome; it's the shit! I'd say the same thing about Family Guy (I like that show as much as The Simpsons; they're both equally good). Once the show hit its fourth season, Puritanical morality flew out the window! Obviously, any show on TV had to (and still does have to) obey the FCC and Fox's censorship policies, which made it difficult for The Simpsons' creators to get the adult content within the episodes to air. I for one do find television's censorship policy a bit strange and hypocritical. Of course, The Simpsons had sequences that were forced to be deleted during its original broadcast, but thanks to subsequent broadcasts and DVD, you can now watch all the episodes in their uncensored entirety! And thanks to the internet there is FAR less censorship, which is good! Now, I'm not saying that censorship is ENTIRELY bad. It's important to have SOME censorship in order to prevent the worst that could happen in our lives. Everything in moderation. No pun intended.
I for one have a problem with The Simpsons' young fans (children and underdeveloped teenagers). I mean, I think fandom among younger audiences should be accepted but discouraged. The reason I think it should be discouraged is because children and underdeveloped teenagers are not stable-minded enough to associate themselves with it. The franchise, but the show in particular, contains disgusting violence (especially in the Itchy and Scratchy segments and Treehouse Of Horror/Halloween episodes), sometimes callas jokes, borderline-pornographic sequences, sometimes disturbing sequences, illicit-drug use and cursing. Definitely NOT "family-friendly". Obviously, South Park is FAR worse in terms of its offensive/adult content, even Superjail or Drawn Together are in that category of being a bit extreme. But let's face it: The Simpsons is not for little kids or for those whose minds aren't mature enough. Nobody should be watching it or having anything to do with its fandom until they are of age.
I know I'm being a bit of a hypocrite here, but I started watching The Simpsons when I was 12, even though in retrospect I was "underage" or too young for it. I think I might have been exposed to it when I was young as 5. That's even worse! I was in Kindergarten for fuck's sake!
I hate it when people call The Simpsons a "family show" when it really isn't. It never was. Sure, there were and are kids and families that watched it and still do to this day, but it really wasn't/isn't meant for that audience. It was and is primarily intended for adults. I would say it's R-rated, although certainly not NC-17 like South Park, Superjail or Drawn Together! NC-17 is FAR worse than R! The Simpsons Seasons 4 and beyond make Seasons 1 to 3 of it look (almost) PG-rated.