This article explains growth plates pretty well:
Exercising Growing Puppies - FamilyEducation.com
Two years old is plenty old enough to start agility. Puppyhood is great for training the basics, sit, down, and stay, which are necessary for agility. Tricks are also great to train, and I like having them to do while waiting at the start line. I feel it keeps me from getting too nervous, warms up the dog and gets her focusing on me.