I train all three of my dogs in the sport, Smudge and Winter both compete and Tama still has some kinks to work out but really agility is a sport that you always have to work at to improve both yourself and your dogs.

What you need for agility is a good relationship between yourself and your dog, you don't really have to worry if your dog doesn't have all that much confidence that comes in time, trust me on that one. The dog should have a basic level of obdience like a good recall, a sit and down, and a decent wait or stay. It's not always necessary but it's also a fairly good idea that your dog is at least tolerant of other dogs because in a class in environment a dog can get away from it's handler and then all kinds of mayhem can insue. Other then that all you really need is a good attitude towards the sport and a good teacher. Agility is not the type of thing that you want to attempt to teach by yourself as there are some safety issues with contacts and some of the other pieces of equipment that should be taught to your dog by a professional.

As for age the classes that I help out at will allow a dog over the age of 6 months in to the beginners classes but we do not allow any of the dogs even those that are over two years of age to jump at full height in the building, any jumps that we have are set at about 2 inches of the ground so even a puppy can do it (the training centre that I go to also runs puppy agility classes, where all pieces of equipment are set at uber low heights for the dogs) But I warn you young dogs under the age of 2 should not jump full height and they should be watched so that they do not jump off of a contact piece of equipment.