The following is a contribution by Hendy Lund, who is by no means a professional trainer, but who has a very energetic puppy. Here are some of the things I'm working on with my puppy, Zadi.
Agility requires several skills, only one of which is executing each obstacle correctly.
Equally important is the ability for your dog to follow directions, work at a distance from you, and perform a series of tasks in the correct order. These skills you can start teaching at any age. The single most important thing you can have when starting agility is a solid recall command. On the agility course, your dog will have to be able to work at a distance from you. You can use any word you like, keeping in mind that you'll have to gasp it out at speed someday.