I've always given Jackson people food (but nothing terribly bad) since he was a pup. He's now 1 year old. It's never really affected him. Obviously stay away from chocolate, greasy foods, and yucky things but real chicken, real steak, I use bits of cheese sometimes for training, tiny tiny bits of hot dog for training, etc. He's never had stomach issues. My vet told me that as long as you give them small portions - it's fine to give a dog people food. He's not overweight at all. Granted - sometimes I wish he had never had people food before because he's kind of a beggar but we're working on it.
