Nowadays they need to start young (5/6) if they are going to be competitive but the big risk is burning them out by pushing too hard.
I know a family that have 2 lads 9/10 and they swim 6 times a week and do a gym session as well. This is too serious at that age IMO but the coaches are playing a numbers game and don't bother about the attrition rates as long as they manage to get one good international.
My son is pretty handy at DH, I basically help him to ride as much as he can/wants, make it fun, encourage him to develop his skills and do other stuff that will help his physical development. I won't force it and make him train because that's something he has to decide to do himself for it to actually stick. No matter how pushy the parents are, if the kids heart isn't in it he/she will never get really good. I was a fairly good climber back in the day and this is pretty much how my parent dealt with it for me.