Puppies can begin eating dry hard food once they begin growing teeth and once they've been weaned off the mother dogs milk and breasts.
The puppies will usually begin weaning off the mother dogs breast and breast milk by 3 to 4 weeks of age and then once they start growing enough teeth then they can start eating dry hard food.
Some puppies are ready to eat dry hard food by age 6 weeks but some puppies may not be ready to eat dry food until 8 or 9 weeks of age.
But if they're not ready to eat dry hard food you can feed the puppy some wet canned dog food until they're old enough and ready to eat dry hard food from a bag.
But as soon as the puppy gets it's teeth in the puppy is almost always ready to eat dry hard food but can certainly eat wet dog food in a can.