LOL if you want well bred dogs as a teen, get into doing dog stuff and breeders will be offering you dogs left and right
For me, age isn't a concern really. Probably because I have been involved with dogs since I was a kid, teach 4H and personally know a lot of under 25 (since that was the number someone mentioned, I'll go with it) people who are great homes. I have never been a fan of blanket policies about age, kids, fences, etc.
So the answer to the question is that it depends on the person. Definitely some people are weird about all sorts of stuff when it comes to selling puppies. There's people who won't sell to you because of your age, what you'll feed, if you don't have a fence, etc, etc, etc. But for the most part, breeders want good homes for their puppies and if they think you're a good home the age shouldn't be an issue for most.