most places it's doctor's call. depending on what doc you talk to and what clinic, you'll get different ages at which they'll do it. it's basically doctors discretion most places.
unfortunately, what you may end up running into (because i dealt with it & have known many other women who've gone thru the same) is when you're 18 & asking, they'll say 21. then, if you're 21, they'll say 27 &/or 2 kids. at 27 they'll tell you basically "well, what if you still change you mind?"...
it IS possible to find someone to do it, but it could be very hard finding that person. good luck.
i know one particular woman who is 37 now, has 1 kid, and doctors say it could be life-threatening for her to have another -but no one will give her a tubal. not for health-related reasons, but for the same old "don't you want another child?" one. sigh.
sorry, this is a sharp point for me. keep looking around, talk to PP, someone should be able to do it for you, even if you need to go to the next state. good luck.