I think Koke started with her knee problems between 8 and 9 months of age. She had the right one operated on in April and we are going in to see about having the left one done tomorrow
I'm not so sure you should give glucosamine to a dog that doesn't need it. My other chi, Sandy, doesn't have any knee problems and I asked Kokes surgeon about giving it to Sandy and he said that it was not a good idea because it can cause their blood to thin out. So, I would suggest waiting until your pup needs it. I do give it to Koke though.
As far as changing the food, I think the vets usually recommend changing foods at 1 year old as long as your pup isn't having any problems, but a few months sooner shouldn't hurt him.
Good Luck with your neuter!