Any goat can be trained on a leash, its the person who have the patience and treats LOL! Do not let them do what they want to do (eating, stopping for no reason at all, etc). I never had any problems training at any age. Sometimes some treats in hand helps a bit too! Make sure the collar is snug on their neck, because if they pull forward hard or back up or balking, you can be sure the collar will slip off.
I had one stubborn doe who refuse at any cost except when I had a feed can with her favorite treats, rattle it when I want her to go. "Walk" and rattle can would suffice.