At what age can a puppy eat salt?

You need not supplement salt for your dog. Dogs need very little; kibble usually contains a small amount of salt, not for flavor but to supply sodium, so it barely tastes salty. Kibble already meets the need, so do not add salt, and homemade food needs no salt or other seasonings. Too much salt harms health, causing hair loss and dehydration, and in severe cases kidney failure.