the part where their breasts are still growing is tricky, but i don't know. i don't think it's all that rare for a girl to reach whatever size she's going to be at around 16-17. i wouldn't use that as a basis for legislation, though! also, if a girl is insecure at 16 because of things like that, what's going to change between 16 and 18? unless she just suddenly matures into a VOLUPTUOUS WOMAN then the problems are still going to be present; these aren't just things that go away if you ignore them. and then, i mean, yeah they cost a few grand but the assumption is that if you're considering getting them, you're probably not the kind of family that's hurting for money to begin with. i think all types of implants (the ones that go in from the bottom, and then the ones that go in from the nipple) leave SOME type of scar, but it's honestly incredibly minimal and the health hazards like having them burst are exceedingly uncommon, i think!
but it's not just about WILL SHE GO CRAZY if she doesn't get them, it's about whether or not it will improve the condition of her mental health and self-image if she gets them. even if she doesn't go insane without them, the psychological issues a lot of women get implants to combat can become serious problems not too much further down the road, and ultimately i don't think this is something that can just be solved with counseling all the time. you're not covering up the problem with something cosmetic, because the problem STEMS from something cosmetic. therefore, a cosmetic fix is probably as close as you'll come to a solution, i think! ideally people wouldn't need to do this type of thing but i think it's unrealistic to expect everyone to just be like FUCK THE MEDIA IMMMM BEAUTIFUL.
like the op said, this is getting fairly common in certain circles, so even if they'd be mocked in your school, the odds of that happening in any school a person getting this done would be going to are much lower. having parents pay for cosmetic surgery is nothing new among the upper class, so while it seems ridiculous to you, it's really not.
Is it medically safe to have breast implants while the breast is still forming and growing? Are adolescent views fully developed? Are adolescents capable of making lasting decisions? Do adolescents truly understand societal pressure, or is high school pressure a completely different beast?
The safety of having it done when you're still developing is a good point, but again, I don't think that's universal. As far as the questions about what adolescents are capable of doing and understanding, do you really think you're much less of an adolescent at 18 or 20 than you are at 16? You're not. It's an arbitrary age that we just use as a marker for adulthood, but it's not as if you're suddenly an adult. I've met very few people below the age of 25 that I'd even really call ADULTS, to be honest, so all the questions you're asking would apply to an 18 or 21 year old as well.