As Rahvin said, so long as everyone involved gave consent and no one is being greviously hurt, then I see no reason to tell them to stop.
Would I want to be involved in anything someone can come up with that falls within those two boundaries? Absolutely not.
Again, consent is required, so anyone not mentally developed enough to do so cannot do so. Where exactly that line is depends on the person, but the lines various states make between 16 and 18 is a good place to start.
If someone would really get off by having their arms broken, I think I would draw a line there, too, which is why I said grevious hurt above.