The age for a dog to start competing at is 18months old for all the organizations. As far as the age for classes go, even puppies can do it as long as it's not putting a lot of pressure on their joints. In otherwords they shouldn't be doing a lot of jumping but they can still learn how to do the tunnel, chute and get a slight start on some of the other pieces of equipment. It's also a good chance to get an early start on the flat land training (Ie. following the lead hand.)