Well, I am not sure about the age but when Jane was young we changed from puppy to adult food by first mixing in small amounts of adult food in the puppy.
Then larger amounts in time until the get used to the taste, then put smaller amounts of puppy food and larger amouts of adult food.
And after a while your furbaby will get used to the adult food and then you do not have to mixed in the puppy food. IMO
Hope I help!
