I have 5 tattoos, most of my friends have tattoos. I got my first tattoo at 16, and am planning my 6th soon. That said, that first tattoo, I got it covered up at 25. Everyone I know who's gotten tattooed has wound up covering up the ones they got really young. Most tattoo artists I know have a policy not to tattoo anyone under the age of 18, even with parental permission, because sooooo many people regret those young tattoos. Even if you go on to keep getting tattooed, having an early one you regret might get in the way of a bigger and more ambitious tattoo plan later on (I didn't plan my quarter sleeves well and now I'm struggling to try to integrate a half-sleeve plan with the tattoos I currently have.)
My ex and I used to have a song we sang to my nieces when they were little "No tattoos 'til I'm twenty-two." I've just found that tattoos done before at least 18 or 19 tend to be far more regretted than when they're done even a little later (I know one guy who got his favorite album cover tattooed down half his arm. It's not his favorite album anymore, ooops

: )
Strangely enough

my ds has grown up around tattooed, pierced, scarified people, and he has absolutely no interest in getting tattooed. If he did, I would give him permission (I'd be a little hypocritical telling him about my teenage tattoo), but I would ask him to wait and think about the design and placement and mull over what it means to him and why he wants it. It does bother me to see kids with tattoos, but mostly because it bugs me to see tattoos becoming a fashionable trend. For me tattoos are for important events, or are a transitional experience in themselves, and I just don't think that most 15 year olds are thinking much beyond how cool a tattoo will look