Usually you switch between 8 months and a year. You can call your vet or shelter/breeder and ask though. My mom breeds dogs and she keeps everyone on puppy formula no matter what their age. Something about the calories and nutrients being higher than that of adult dog food.
I kept my dog on puppy food for about 2 years.
She always ate grain-free, always received human-grade supplements (such as hard-boiled egg), and for about 8 years in early adulthood ate BARF (raw meaty bones and raw food).
She's almost 20 years old now and seems to be about 12 years old in her energy, appearance, and alertness. No cataracts. No health problems whatsoever. Perfect (selective!) hearing.
My dog is a terrier. With your larger breed, I would give puppy food until 16-18 months. (Edit: larger breeds don't have the long lifespan of terriers and could become overweight on puppy food into adulthood - unless you fed them less).
The fats and proteins in puppy food build the dog's skeleton, connective tissue, and organs. It is your investment in the future health and longevity of your dog. Now is the time to ensure that you don't face a future of chronic illness and degeneration, as well as medical bills and pain and suffering for you and your pet.
Maybe at about 12 months you can start giving one third adult food mixed with the puppy food.
Six months is way too young to switch to adult food. In particular, boxers need to have excellent skeletons especially for facial and spinal/pelvic bone development.
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