
If pup going before 12 weeks it needs nothing more than a vets cert for the airline to show it is fit to fly.
Don't know minimum age, but I have had pups fly in cabin at 8 (Europe) and 9 (USA) weeks a few years ago, and 11 weeks Cargo to USA in May last year.
it is often cheaper if the new owner can get a budget flight to fly pup accompanied (even if they are in hold) as then they only get charged as excess baggage $150 standard one way charge with American Airlines for example, as opposed to around £500 for a medium size varikennel and pup as Cargo..