I adore this show. As it's been said, Shin is mischievous in ways similar to Bart Simpson, but he never means it in a malicious way. Shin is simply a silly little boy.
To me it's not Shin's exhibitionism that makes this show 13+, it's that when the writers re-write the script for Cartoon Network's [adult swim] programming, they modify the dialogue or story to be more dirty than it actually was in Japanese, which is part of what makes it so funny in my opinion.
A lot of the young characters in this show seem to go about thing with a bit of older then should be and child like innocence of just over hearing the adults, that bring this comedy to a realistic start. shins exposing of him self is the only thing i see to be the bit controversial, but it is true to life, as i remember baby sitting my cousins son at 5 he pop-ed his pasts right off and peed on a near by tree.(much to my dismay).
It's just kids being kids
In my opinion, this show is ingenius. However, it's not for little kids. the main character, Shin, likes to pull down his pants down and wiggle his butt around. occasionally his privates will show, but mabe mistaken sometimes for his bellybutton. His behavior is the reason this show is not for kids. otherwise, it would be suitable for ages 9+.