How old is old enough to get a tattoo?

I say it's when you are on your own and can pay for it yourself. Mothers of 3 my teen daughter's friends have allowed their kids to get one at 13, 14 and 16. I won't be changing my mind about it, but just curious of other opinions. Thanks. (btw, these are NOT small little tats of erflies or hearts, etc. One is a very large scroll with her name on it on her arm, the other is a somewhat smaller one of her name and a heart, while the other is a tramp stamp!)