You are most welcome...
Yes, I would wait because of he'll still bel growing! It could definately cause him to be sore later on in life and probably shorten his pulling "career". Up north, there isn't as much information available about what is best for dog's long-term and those who are truly dependent on sled dogs probably don't want to "waste" time waiting until the right age. Since it would just be a sport/hobby for your dog, I certainly wouldn't want to risk causing any stress on his bones...
It probably wouldn't cause any serious damage to have him pull a light-weight cart around 12 months. But you have a lot of training still to do and I would perfect that before thinking about your dog actually pulling something.
As a pup, I had my dog carry an empty backpack. At around 10-12 months I started to weigh it down and now I have it on him regularly, usually with about 5-10lbs of gear in it - I found that it helped prepare him for pulling! We haven't done carting/sledding yet but we do Skijor - I didn't start having him pull until he was 16 months old (my dog is 60lbs).
"Animals are reliable, many full of love, true in their affections, predictable in their actions, grateful and loyal. Difficult standards for people to live up to."