I don't really like the idea of age restictions because someone who's, say, 18 or 19 might change their mind later. You could be 25 and get your tubes tied and then change your mind before the age of 30.
I absolutely want to get my tubes tied at some point. People can talk to me all day about how I "could change [my] mind down the road," but I've always been a strong advocate of adoption so if and when I decide I do want my own children, I'll take the road of adoption. I'm totally cool with that.