I think it is good for a child to socialize. At what age? Well, that depends on the child. A few hours in the morning even at 2 years of age is good,IMHO. Also it may be good for some children to be away from their mothers for a few hours to foster independence.
Now, for a mother to be away from her child all day, NO,IMHO not good. For a child to be literally brought up by nannies or kindergarten/nursery people, NO, IMHO, not good.
Of course there are cases where this in unavoidable due to circumstances of the parents. I had my child once I could make sure that I would be able to stay at home.If my circumstances would have remained so that I can't stay home, I would not have had a child. As I believe the welfare of the child is ALWAYS first priority and comes before my or my families wishes.