Dunno about anywhere else but the one local to me are very rigid over this law too. My eldest was a few weeks off her 18th, and I went with her get a tattoo she wanted done (her 1st- obviously!) and even with me being with her and claming she was 18 (I know!!!) he refused to do it without photo ID with DOB on.
If you are in the UK then it is 18 with proof of age (photo id) like Boo said!
However if you were to go to a holiday destination they seem to do it at any age.
My only advice is find a good one that plenty of people have used and you can see their tattoos. I used to live near Swindon and went across there to get it done, the staff were all really clean and so was the place.
although you are supposed to be 18 to have a tattoo done without parental consent my daughter had 2 done when she was about 15. The tattooist knew how old she was and still went ahead and did them.
My daughter was 19 when she got hers done, she wasnt asked for ID/DOB even though she was quite young looking. Any reputible place will not tattoo you even with your parents permission unless they know for sure you are 18.
I have no intention of doing it, nor of going with anyone else to get it done, so don't any of you worry about me. As per usual, it's research for my novel. My character, who's fifteen, will have a tattoo applied when he's old enough, to indicate his membership of a secret society.