6 months is the usual age. Unfortunately unless you get a medical waiver from your vet, the shot is required by law. If it makes you feel any better, my two have never had a reaction to the rabies shot. They got the three year one when they were about 6 months old, and have had it twice more since then (they're both 6) The safest thing to do would be to make sure that #1, your pup is in good health at the time of the shot, #2 Have ONLY the rabies shot that day and no other vaccs two weeks before or after that, #3, stick around the vets office for about 1/2 an hour after the shot and watch your baby for any reaction. If there is a reaction, you'll already be at the vets where they can help immediately.
Good luck and do discuss your concerns with the vet
