I had the same problem! My pup Lola use to eat
Puppy Chow because that is what her breeder fed her, but then our Vet recommended
Royal Canin. We got RoyalCanin for Yorkies, Lola LOVED it! But then we realized she had to be 10 months or older to eat it. It said "
10 months+" on the bag, I don't know how we missed it?

Anyways we then had to switch her food again, buying Royal Canin for puppies. aye yai yai!
So I say 10 months?