For boys, I think the BIGGEST change after puberty is the development of a more prominent jaw and chin. All of my male friends in highschool (especially my best friend) who didn't have a strong jawline looked TOTALLY different after abt. their sophomore year in college. They suddenly all had jaws! I think it's got something to do with hormones. I would wait longer than 18 years -- I think you would be smart to wait until you're 20 or even 22. Definitely wait. Plus, (and this is just my opinion) I really haven't seen anyone look good after a chin implant. It could just be the ppl I've seen, but I think you should be cautious.