The main problem I have with such things is the young brain simply hasn't finished developing. No matter how mature they are or act, they simply don't have the cognitive abilities that they would when they are grown. To push your kid into such dangerous things that they would be better prepared for later in life doesn't seem like good parenting to me. The ocean/mountain/whatever will still be there, and there are always records to be broken.

That said, I feel even more strongly about telling people what they can and can't do, and I don't think anybody should be told how to raise their kid short of blatant disregard for the kid's health.

Also, if parents are willing to let such young kids do this, they should be able to fully pay the bills for any rescue needed, and able to compensate families if a loved one is lost trying to rescue the kid.