Here is a sticky thread from the Breeder section that you may find interesting and useful. It breaks down the development of a pup's skills by each week.
http://www.yorkietalk.com/forums/showthread.php?t=31003
You will not find many respected breeders that will let pups go any earlier than 10 weeks...and that is pushing it. This is the advice and opinion of most SUCCESSFUL, long time breeders. They really know there stuff. Rest assured, keeping a pup an extra month is not a bed of roses for any breeder, so there is good reasoning behind it.