At what age can puppies be sent to America ?

Over 8 weeks, though if they have been vaccinated for Rabies they can't go until after 21 days to EU countries so likley the same or similar.

I have had both a puppy go to the USA and come here (obvioulsy quarantined in latter case) at 9 weeks.

There is info here: http://www.aphis.usda.gov/import_export/animals/animal_import/animal_imports_pets.shtml

The Centers for Disease Control and Prevention (CDC) has regulations on the importation of dogs and cats into the United States. In general, they require that dogs be vaccinated against rabies at least 30 days prior to entry, except for puppies younger than 3 months and dogs originated or located for 6 months in areas considered to be free of rabies.

Thanks , thats great.I have a puppy from my next litter that will go to MI in America , wont be for a while but want to get as much info as possible