I waited until King was fully grown. I probably should have done it at about 1 year, but I have had friend's dogs that had it done very young, and seemed to change so drastically (personality, weight gain, energy level in general) that I waited until he was almost three. My friend lost his dog during neuter surgery about the time King hit the year mark, (some oddball reaction to the drugs used to put him out), and it freaked me out. Happily, other than making King slightly less muscular looking, it didn't seem to affect him in any way. My vet thought I waited too long, but it all worked out OK.
Even if I had done it at a year, King still would have fathered the 8 black and white pups he did when the neighbor's Lab/Husky mix jumped the fence when King was about 9 months old. All eight are alive and well, and about to turn 8 years old in November. Sadly, mom Goofy was hit by a car when she got loose again when the pups were about 6 months old. She was an escape artist and almost impossible to keep in the yard, or in the house. It was very sad.