Most puppies should start developing teeth by the time they are 3 weeks old. You can start introducing soft puppy food to them at that age but make sure they still have access to mothers milk until they are 8 weeks old or until mother dog weens them herself- which may be anywhere from 6 to 8 weeks. Make sure to provide plenty of fresh water to the pups as well so they do not get dehydrated. You may want to offer hard puppy kibble as well. They will naturally become curious about it and you will find out that hard food helps the puppies produce firm stools which will be easier to clean up. Most people are not aware of this, but make sure to get your puppies dewormed when they are 2 weeks old. Many puppy deaths can be attributed to worm burdens. Good luck to you!