I have been watching the show since I was five. The show can get a little sketchy, but for the most part, it's fine. Sometimes Marge and Homer do have s*x, but you don't see them doing *it*. If your kids are not familiar with drugs and alcohol than I DO NOT suggest you letting your kids watch it. Also my little brothers are 6, 8, and 11, whenever they watch an episode, my mom makes sure that nothing "Bad" leaks out. I recommend having person over 12 watch the show with a younger child.
Make sure you talk to your child about what might happen during show.