Goalkeepers need to do there own specific work away from the group for a period of the session. For me they do the warm up and opening drills getting the eye in. From that point the 2 keepers if there is 2, should break away with 1 of the coaches and work on aspects of the game keepers need to excel in. After about 20 minutes, or longer if needed, they rejoin the main group.
As an aside I think the same should apply for defenders and forwards, break away in their groups and drills specific to that role.
Finally going back to your second question, personally I’d start it at 14 before thatI’m not sure a team should have a set goalkeeper, all players should play all positions until then in my opinion. But from 14 on a defined keeper should be chosen and drilled correctly in the position.