I think that once you are done with the rapid growth phase (6-12 months of age depending on breed) then glucosamine is probably okay.  I diagnosed Loki with minor hip dysplasia when he was 14 months old and started him on Dasuquin without a problem.  He is almost 6 years old and so far he hasn't had any hip pain. 

Wyatt is old enough that he could take glucosamine to prevent future arthritis (smart for anyone with a tripawd).

High dose vitamin C can help dogs with cancer or existing arthritis but probably doesn't prevent hip dysplasia or other developmental disease.  I do agree that it shouldn't hurt but may cause diarrhea when you first start it.  If your dog has arthritis  then the antioxidants all work best if given together, since they all have a different function in helping to reduce inflammation.  The best combo is Vitamin C, Vitamin E, SAM-E, and fish oil.  Selenium can help but that one you have to be careful not to overdose because too much is toxic.

Pam