It does depend on the size of the breed - essentially they need to be fully grown - smaller breeders one, medium around 15 months, larger breeds 18 months. Those are guidelines most agility trainers go by, but there are no hard and fast rules. I used to train at a club where they were allowed from 6 months, although it was more getting used to the equipment and learning commands. Poles were on the ground, the dog walk and a-frame were flat on the floor etc.... I think this definately works, but not all clubs offer this.

Have you had a look on agility.net?