If it was an actual rule, Deezer would just send a letter saying "No, your kid can't join until he/she is 13, come back when you're older." So, someone can't really be banned for being too young, just that the underage people who are banned just generally turn out to be morons.
Still, you can get idiots of all ages; being older didn't stop Fairemaster from being banned, for example.