All the kids I train is generally between 9 and 12 and all of them have watched South Park which is worse than Simpsons.
I let my 11yo watch South Park, but that's because I am a bad parent.
My feeling is that you've got to look past the naughty words and make a judgment about the values espoused by the programme. South Park is just puerile nonsense - an 11yo is the perfect audience for it.
Pcolbeck mentions Family Guy. I wouldn't let him watch that, but mainly because I think it is vile and misogynistic, not to mention unfunny. I have no problem with so-called "offensive comedy" (I love Curb Your Enthusiasm) but I find Family Guy witless.
To get back to the OP, I am happy to let my son watch The Simpsons, but you really have to make a judgment for your own children based on your own values. Maybe watch it a few times yourself first to get a feel for it. I'd say that the episode you refer to is fairly typical.
Personally, I'd rather watch King Of The Hill than the Simpsons, though I know I'm in a minority with this. But King Of The Hill is wonderful family viewing - definitely safe for a pre-teen but with very grown-up themes to its content.
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