Usually quite young if they are that way inclined, however not all dogs like catching or fetching come to that, my last German Shepherd Never retrieved anything, I tried but he would watch the Frisbee/ball (or whatever I threw) go over his head and land further up the field, watch me walk up the field to collect it and he would then watch the same article going over his head the other way down the field! he had much more important things to do, like looking for rabbits etc, you could see it written all over his face 'well, if it keeps her happy' lol! I lost him a year ago aged 13years and 2 months and I still miss him, what a character!!