I have always kept mine around 15-20 wks. old just b/c by that age they are already; Crate Trained, House Broken, Leash Trained and used to their Daily Grooming Routine; Teeth and Hair Brushing & Face/eye washing and have had all their Vaccinations and Micro-chipping etc. I find that the older they are, the more well adjusted, balanced and Independent they become.
I know alot of Breeder's that sell them very young @ 6 wks so they appear
smaller for their age w/c they lie about by saying they are older @ 8 wks instead of 6 wks.