Being a teenager is already difficult enough without having your parents controlling or stopping you from doing all the things you want to do. That’s why many people wait till they’re eighteen to make big decisions or undergo body modifications, as they’re officially considered adults at that age.
That is also why a woman went behind her sister’s back to help her niece, struggling with her body image, to get a tummy tuck as soon as she turned eighteen. Unfortunately, the teen’s mom found out about it and went nuclear.
More info: Reddit