I was wondering the same... but will say, if the instinct is there, it's easy to arouse. My pups are typically retrieving toys, bird wings, etc at ~5 wks, and part of my litter evaluation here has them on live pigeons at 7 wks. If the desire is there, it's there very very young but I've also seen folks that didn't nurture it and the dog had no desire later. Balls aren't something I throw for mine, so I may have some here that aren't ball driven, but they sure like their bumpers and birds!