My 10 year old granddaughter can make well thought out decisions even to show awareness of the consequences, like don't touch the stove or it may burn ya. But that does not mean I let her go and pierce up her body.
As a parent I think it is your obligation, no your duty to put a hold on things like what your saying, even if they can make sound choices. Teens and young teens especially only see whats up today, they can't truly see how much they will or will not like what they have done to their body until it is to late.
It's what everyone else is doing, I want to be cool, but guess what, that tats I got when I was 14 I wish I never did that. And I did it against my parents wishes.
But that is just me.