Depends on your jurisdiction. In most places in Australia, a parent or guardian is absolutely able to provide consent for their child to get a tattoo. Good luck finding a decent tattoo artist who’s willing to take on the commission though.

I've known plenty of underage girls with tattoos. One told me that she got hers at parties where a bunch of kids got together and paid a pro to come to the house and give them all tats. I know for a fact that she wasn't a rich kid, so it must not have cost that much. Her tats were nice too, not like it was some amateur experimenting on minors.

Questionable Content correctly pointed out that there is no minimum age to purchase a tattoo gun.

But technically it says generally.

My sister has piercings and a tattoo and she's 17 i think. Some shitface asked her out at a gym because "no way you are less than 18, you have a tattoo"