It's not really a question of how old you think someone should be anymore. The majority of US states have passed legislation that mandates that someone must be at least 18 years old to be tattooed, or to get piercings below the neck.
How old should a tree be before it is chopped down?
How long should you leave it before changing your bed sheets?
How long should you shake your penis for after peeing?
On a serious note, I think that it should be mandatory for every female to get a tattoo at the age of 21 years and 123 days. For males, it should be 22 years and 236 days. Anybody who exceeds these limits should be branded on the forehead and outcast from society.
I think a person 18 to 21 should have to submit a proposed tatoo for one year, if at the end of that year they still want it then let them get it. chances are at the end of that year they will have changed their minds. And if they change the tatoo then they will have to wait a year again.
I believe that any adult should be given the freedom of choice, each being should have the liberty to persue whichever path they wish. In the UK, an adult is 18. The idea of an 18 year old having to submit a tattoo proposal for a year seems ludicrious to me.... let them tarnish their bodies if they so wish... all it would do is drive a pretty dangerous industry underground. If an 18 year old wants a tattoo now then they will get one now, they aint waiting until they are 19.... the result is potentially unhygenic and entirely unregulated underground studios giving Hepatitis to your kids...