However, there ARE this world's future dancers practicing their brains out right now, and most of them go through stress. They do things to their body that will make them grieve forty years from now, and it's not even 100% guaranteed that they'll have a ballet life. Sure, they spend thousands of dollars a year on training, throw their education as a second-priority thing, but the ballet world is not generous. It works in a small-pond-big-fish way.
And what if he/she has an accident? The thing that prevented my teacher from being part of a corps de ballet when she was younger was that she had an accident and if she kept dancing, she would have never walked again.
Ballet is also a huge commitment, with plenty of sacrifices. Not to mention the costs for classes and getting ACCEPTED into these classes. I know a pretty damn famous school that doesn't care how well you dance, and only focuses on the body shape that you were born with. You have to sleep ballet, eat ballet, breathe ballet, everything is ballet.
So, in conclusion to your answer, I think ballet can be harmful and risky if you choose to take it really, really really far, but harmless if you take it just to do something with your time.