I've known of some border collie breeders that start teaching simple commands as early as they can see and walk. According to them, it goes wonderfully and gets an early start on the pup's training. At 8 weeks their pups leave knowing sit, down, stay, shake, and a couple of other commands I think. The earliest I've ever started was 8 weeks, which is when I got a puppy a loooong time ago and no longer have.