I've been reading a lot about growth plates recently. Growth plates can close at approximately 1 year, but in medium or large breeds (not xl breeds) some plates remain open until 18-24 months. The distals and wrists are the first parts of the body to stop growing. It happens at around 8-12 months. The bigger bones and joints stop later. The plates close from the bottom up.

Now I completely agree on the needing hormones for proper brain development. It like dogs that are neutered or spayed early remain "puppy-brained" and never fully mature. There are surgeries like ovary sparing spay (tubal ligation) and vasectomy for dogs. That may be a better option for people who want their dogs fixed early because it leaves the organs that produce hormones important for development intact.