When kids are 8,9,10 they are already trying to jump their bikes over all sorts of things, riding them down steps or even the metal handrail. Before long (or at the same time) they are doing the same thing on skateboards. They will ride the skateboard down one side of a "half-pipe" (or whatever they call it) and up the other side launching themselves high in the air while twisting and spinning before coming back down. They do not always have a smooth landing.
They also start doing the x-game type things on motorized bikes while still quite young and each year the things they do get even more extreme.
Some of the divers at the Olympics are in their teens. To get that good at the 10 M platform (that's nearly 40' above the water) they did not just start last year. They have been diving like that for several years. Let's see - in the Olympics at 16 with at least 4 years of training means they were throwing themselves 40' down into the water since they were 12 or younger. And, when they started, they were not landing the dive as smooth as they are now. According to one of the commentators, they hit the water at 35MPH.
Kids are playing tackle football from Pop Warner on - sure they where pads, but they do not prevent concussions, as has been in the news lately. Plus, when they live where it is Hockey or Soccer, they are not wearing all that much protective gear.
But, since these things are not "tractors" no one seems to mind or argue that they are too young to be doing/learning/trying them.