What happens if you feed a puppy senior dog food?

Please don't do this! Puppies need a lot of vitamins and a good healthy start while they are growing and developing. They also need food that will provide more fat and carbs to burn. If they don't get it, they will seem sluggish and won't develop as they should. Senior dog food is made to have a lot of that stripped out because older dogs don't get much exercise and you don't want them to get fat.