Personally I think there are some shows on Adult Swim that are just not appropriate for human consumption regardless of age. Some of it is fun, some of it is only funny if you're stoned on something (not speaking from personal experience here, just impressions.) When it comes to deciding whether or not to let your child watch something though, it really has to come down to your beliefs as a parent and the maturity level of the individual child. There is no "one age fits all."
PG, for instance, is "Parental Guidance Suggested." Your parental guidance may me to say, "This is fine for my kid," and that's valid.