Hope this helped.
If I may be plain, do you mean to possess one or to have one professionally done? Because I had my ears all the way done up by the age of 9. I also had friends who had their noses and navels PROFESSIONALLY done (Several in fact) by the age of 12.
I'm also in California however, so thats normal HERE, whereas in another state it may not be.
What's acceptable I think varies depending on your area, as far as how old you have to be before you can legally HAVE a piercing (not receive one) I believe you have to be over age 8 to have any EAR piercing besides earlobes, or they consider it child-abuse.
Over age 8 it changes because theirs kids like me that simply pierce their own.
Nose, navel, etc, I believe is 11+
so I guess yeahh 12 I wude go lowest !
21?? You can get ear peircing ANYWHERE ON YOUR EAR when you're 16 in the US you just have to have proof. Under 16 then you need a parent.