If it's something my child was sincerely interested in, I don't think I'd bar them from it
I'd wait until I considered them old enough to understand the risks, explain the risks, and let them make their own decision.
None of this Pee Wee 10 year old football stuff though, that's where I think I draw the line. I suppose the hard part is figuring at what age can the kids understand these risks they're taking, but 10 years old cannot be it.