Well… I don’t know if anyone knows the real answer to this because a lot of people don’t admit to getting “cosmetic” surgery. Braces is a form of cosmetic surgery a lot of the time… and a lot of people have done that at 12 years old.
As far as celebrities go, I know Soleil Moon Frye (played Punky Brewster) didn’t get plastic surgery, but she got a breast reduction at a really young age. I don’t think she was even 18. But that was “cosmetic” surgery.
There may have been other celebs that possibly suffered an injury in an accident and then got “plastic-corrective” surgery… but in general that doesn’t happen too often with people under 18, because physically women can grow and develop until they 19 years old… and men until they’re 21 years old. So often doctors don’t want to perform these types of surgeries too early, because you may not know what you really have yet if you are too young.
I fractured my nose at 9 years old and wanted surgery, but doctors talked us out of it because they said I had so far to develop who knows how my nose could turn out to correct itself as I mature. The advice was to wait and see… give it time. I never got the surgery.