They should not be weaned before 8 weeks, and longer is better. The only time I wean that young is when it's a buckling that is staying intact and acting alittle to mature if you know what I mean.
I let sale kids go from 10-12 weeks depending on the kid. For keeper kids, if they are dam raised I leave them on for atleast 12 weeks, usually longer (I may not separate til I rebreed their mom, so like 7 months, as long as they don't seem to be pulling mom down). If they are bottle raised I may start the weaning process as early as 12 weeks, or not til 4 months depending on the kid!
I know different breeders have different idea or methods, but I think most can agree not before 8 weeks! And even though I think that's alittle young, I respect a breeders descision to do it then, but I strongly disagree w/ anyone that would wean earlier.