I have done both flyball and agility and have never had any problems with joint troubles with my mob my old collie is now 17yrs and runs around like a youngster. I never start training until they are a year old if a heavy dog i wait until 15 months or older.You can work on obedience or ground work.When i first started agility dogs were allowed in the ring at a year old and there was a lot of shoulder troubles which is why they made it 18months before you can enter a show and you cant train until a year and the bones are fully formed.Flyball is great fun so if you want to go for it bu
t also make sure that you let him warm his muscles up before he does his first run usually clubs mkes sure they do a couple of recalls then run back from the box. Find your local team and have a talk with them and go and watch a training session. Take ear plugs its really noisey the dogs love it and you dont have to be as fit as you do in agility!