As long as it doesn't involve major piercings, surgeries, or horrifying tattoos, then I say it's fine to let them. I mean we never will never know how we will look unless we try out new things, people will either hate it or love it, for example, I had pretty short hair but I wanted to cut it a certain style everyone thought would look bad on me. Well, I ignored everyone and it turned out they all think it looks very good on me and apologized for saying otherwise. So the best way to go around this is to let them do it, because the only way they will learn is from testing and experience.