Hmmm, it's a weird one with the kids. I would let them get whatever they wanted tattooed when they hit 18, but i would advise them against it. If you still want the same in a couple years as you do at 18, then get it. If they were going to get one anyway i would then advise them to get it done on their legs or back or some such, don't waste prime real estate on 18 year old tattoos.
There's also the question of letting them do stuff or not. Mum didn't let me drink when i was 15, of course. So i'd go down to the skate park with my friends and get drunk in public. And get home and pass out on the steps. So i'm of a mind that you should let your kids get used to booze, because they'll f**k off and do it behind your back anyway, and at least in the former scenario you can keep them somewhat safe. And me with three years of boozing under my belt, i wasn't too bad when i could legally drink in pubs and clubs compared to little Johnny No Drinks. And little Johnny No Drinks gets his head kicked in because he is a belligerent drunk, and nobody knew that because he never tested the matter before.
I think it also depends on the kid. My sister is mature, and was pretty early. Me, i'm a silly merkin, and always have been. Then again, i don't hit the drugs because mum would crack the shits, so maybe there is a case for some of it.