When my DD was 3 we went to the AG store in Chicago over Thanksgiving while we were there visiting relatives. We just went to look as I knew nothing about AG at the time. DD was in awe of course

and wanted one terribly but 3 is too young, so she got a miniature Molly doll and a promise that when she turned 7 she'd get an AG doll.
That summer we went to Disney and she discovered the My Disney Girl dolls. Well, when she got home she got a package from "Mickey" (aka Mommy) that had a Disney Girl in it and said "Sarah, this Disney Girl needs someone to love and take care of her. Promise me you'll be good to her." Once we opened it I told her that if she could show me that she can take very good care of her doll she'd definitely get her AG when she turned 7.
The disney girl doll to this day has her own bed in DDs room and is as immaculate as the day she arrived, though she's been played constantly. For Christmas this year all DD asked for was her AG doll because she's 7 now

She was not disappointed.
I think it depends on the maturity level of the child. AG dolls are very expensive but if the child is going to play with and take care of it nicely I say go for it, they are wonderful dolls.