The basis of a long and active dog life is a species appropriate balanced nutrition during the important phase of growth and development smaller breed puppies tend to reach adulthood by 10 12 months but large breed puppies keep on growing up until they are 15 months old in order to grow into a healthy adult dog your puppy needs a specialised food to meet their various nutritional needs when deciding on a food for your large breed junior dog you should take the extended growth phase in to