Our policy was always "dogs had to be over 1 year". We got a lot of pushback, and settled on a compromise. For puppies, we require a pet deposit that is equal to the cost of a carpet replacement (in case of puppy accidents). If the carpets don't need replaced, great. But if they do (we've never had a puppy that didn't leave pee stains) the owners have already paid for the replacement up front, so there's not the shock at move out.