Is it slutty for a 12 year old girl to have a belly ring?
I think the more appropriate question should be “Why would a 12 year old want a belly ring?”. The only reason I can think of would be to show off her belly, which is inappropriate at 12 and could lead to more inappropriate behavior at a young age. Such things are not a good idea until the young lady is more mature and able to handle the consequences of her actions (and reactions to the belly ring).
