Well I am not entirely sure, but everything I have ever read and everyone in agility I have talked to have all said the same thing, wait until the dog is fully grown. I think this is because weaving is the most unnatural thing we ever teach our dog to do, and it requires multiple bends in the dog's spine. Thankfully some agility organizations have just voted to increase the spacing in the weave poles to 24in so it is better for the health of the dogs.
I know Susan Garrett, developer of the 2x2 method, suggests waiting until the dog is closer to a year and a half and doesn't even start puppies on anything until that time since dogs can learn so quickly with her method. Silvia Trkman does start puppies on the channel method, she started Bi at 6.5months, but this is because she spends so much time working on entries with a puppy and she doesn't close the poles until much much later.
But like I said, I am not a vet and am no expert in agility either. But in my opinion I would spend my efforts training other things, and/or re open the slants and really work hard on angled entries, getting independent weaves regardless of your body position, and speed in and out of the weaves. It's a lot of work if you want this independence and speed and I don't think a puppy has the coordination down. Even if you have a great performance as a pup, the striding is going to be completely different when she's finished growing.
So with Vito, 11 months and 18inches, he is "jumping" 8 inches and we are doing a ton of little sequences and flat work. I also am working on his contact behavior (we're doing running contacts), but don't do an actual aframe yet and rarely do the full dog walk. We practice weaves occasionally, but never closed. I just want Vito to know that weave poles are really cool and to be able to find his entry regardless of where I am.
