Does anyone here participate in dog agility with their pup?  What breed?  How old did you start him?

Kind of but not really.  I pimped my dog (Velocette) out to the training facility for kids who wanted to learn how to run dogs through agility courses that didn't have a dog.  This was a 2 week summer camp for the kids.
Velocette is a Sheltie and was about 2 years old when she went into the course with no previous training.  At the end of the two weeks she was able to complete all of the obsticles with no problem and little coaching from her handler.  Out of all of the dogs there, I would say she was in the top 3.  The dogs that seemed to do well were your typical "good" agility dogs like collies or golden retrievers.  Dogs that didn't do so well were smaller, think purse dogs, labs and a severly retarded dalmation.

Velocette running the course


And just cause, herding!