The younger they are the more help they need, however th eapproach I like to take is fairly simple. If they are willing and able to help care for, bath, and prep the birds then they are old enough to begin to show. We have one young man we with who is 7 and does fine but every now and then will stroke a feather the wrong direction, and does need help with coop in, and coop out at the shows. this is his first year showing but his brothers have been at it now for a few years.
I do have to help him with wiping them down and cleaning faces and legs right before he coops them in, his hands just are not quite big enough to handle the birds well and wipe them down or clean up real good, but he tries very hard which everyone present can see. Had several make comments about how hard he tried and although he had some assistance there were no complaints rather the opposite from the youth leaders and parents about him and his birds.