[yorkiemini]

All good agility trainers start the dogs on very low jumps regardless of their size. In competition the heights they jump are determined by how tall they are.
They don't allow very young dogs in agility until, certain growth plates in their body have reached maturity.

Dog are required to know sit, down, and stay before starting agility.

Like anything else one does with their pups both the dog and handler should enjoy it. If not, then it is not for them. I had a friend who's dog was great In obedience, but it was clear the dog did not enjoy it and I always wondered why they did it - for the trophies and ribbons I guess!