http://www.apbt.info/tiki-index.php?page=Beginning+Weight+Pull
This link is awesome read.
I know the dog wp competition coming here starts the competition at 9 months old. But recommend the trainers be professional to start them that early.
Personally??I'd recommend the command training and getting used to dragging light objects such as milk jugs from 6 months to a year. At one year, I will have the dog looked at by a vet to make sure he can start lite weights, such as pennies in the milk jugs, small amounts and gradually increasing. Then, go up from there over a course of a year, with close vet supervision, and no really heavy weights until 2 yeras old, and x rays done again to confirm he is ok to continue to the big boy stuff.
I will not jeopardize my dogs health and structural well being, because I am not experienced and in no way a professional. However, I am getting pumped with good advice since I have taken this on, and am learning tons of info. We are having fun, and working hard!!!
Dont rush it, it will come, and please dont hurt your doggy!