Never before 6 months even longer sometimes,, I dont feel they are ready before that age and have never ever had any problems with them missing their dams because they are that bit older and both are ready to be seperated . If its a filly i will sometimes even leave them in until they naturally wean,,no matter how long that might be, As long as the mare can cope of course..
I dont understand why anyone would wean before 6 month's if the dam was in good heath and not loseing weight but that is just my opinion