You keep using the word maturity like some 12 year old school girl. I'm talking about understanding complexity in humour... Probably more a measure of intelligence than maturity.
Jokes involving bodily functions are pretty low on the totem pole, as far as I'm concerned anyway. Chances are an 8 year old isn't going to find many stand up comics very funny, but will crack up over an episode of Cow & Chicken.
It's not a measure of said 8 year old's maturity level, but their ability to understand unconventional logic, irony, observation, or any number of things that makes a more complex 'joke' funny. They have no problem understanding an odd sound + a disgusting smell = funny, though.