Last year, in an effort to control puppy-mill imports into the USA, both the CDC (Center for Disease Control ) and the USDA (US Department of Agriculture) enacted requirements.
The CDC has rules that puppies must be vaccinated for rabies no younger than 3 months of age, then wait 30 days before entering the USA. So effectively, puppies must be 4 months of age before entering. There are some exceptions, like puppies from rabies free countries.
The USDA is also trying to curb commercial shipments, i.e. puppies that will be up for sale here in the USA, but increasing the age limit to 6 months. typically importers are trying to bring in whole litters or multiple puppies at once.
There are other requirements beyond the rabies vaccination. Animals coming form a country where screwworm is known to exist must be treated appropriately and be screwworm free upon shipment. Working farm dogs like collies and shepherds must be checked and/or treated for foot and mouth disease prior to shipment.
In these cases a customs broker will be needed for commercial entries and custom clearances.
Airborne Animals LLC can help with you pet importation, as well an moving pets out of the country to other parts of the world.