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How old does a kitten have to be to fly?

There isn't one number. The answer depends on which airline is actually operating the flight, and the gap between them is bigger than most people expect.

Search this question and you'll get eight weeks, confidently, everywhere. That's right for some airlines and wrong for others, and getting it wrong costs you a flight you've already planned around.

The minimum belongs to the airline, not the industry

Delta's published minimum for pets in the cabin is eight weeks for domestic travel. American is the same, at least eight weeks within the US and Puerto Rico.

United is the outlier, and it's a big one. Their in-cabin policy requires puppies and kittens to be at least four months old. That's sixteen weeks. A twelve-week kitten that Delta would carry without a second glance cannot fly United at all, at any price, in any cabin.

So the useful habit is to stop thinking of this as an industry rule and start treating it as a per-route question. Two more things follow from that.

First, check the airline that's operating the flight, not the one that sold the ticket. A ticket with a United flight number can be flown by a United Express partner, and a regional operator can have its own limits. Look for "operated by" on the itinerary.

Second, recheck after any schedule change. Airlines rebook people onto other carriers when flights are cancelled, and a kitten that was legal on the original itinerary might not be legal on the replacement. That's a bad thing to discover at the counter with a buyer waiting at the other end.

Two other things usually matter more than the airline minimum, and they catch breeders out far more often.

The destination state can set a higher minimum

Airline policy is a floor, not a ceiling. Several states set their own minimum age for bringing in a cat, and a few set it above eight weeks. The rule that applies is the one for the state the kitten is arriving in, not the one it left. These requirements change, so check the age minimum for your route with the destination state's department of agriculture before you book the vet, rather than assuming it's the same as last time.

Your own contract may set a higher one

Plenty of catteries won't release before ten or twelve weeks no matter what the airline allows, and most experienced cat people would say that's the right call. Eight weeks is legal. It's also early for a kitten that's about to spend a day in an airport, meet a stranger, and land in a house it's never seen.

It comes down to the carrier

People assume there's a weight limit. Mostly there isn't, at least not in the cabin. Delta and United both frame it as fit rather than pounds: the kitten has to be able to move around inside a carrier that goes fully under the seat in front of the passenger. American publishes a twenty pound limit for pet and carrier together, but only for the premium cabin on its three-cabin A321T.

What actually decides it is under-seat space, and that changes with the aircraft. American allows a hard-sided carrier up to 19 by 13 by 9 inches on mainline flights and only 16 by 12 by 8 on American Eagle regional jets. A carrier that's fine on the long leg can be too big for the short one. Size for the smallest aircraft on the itinerary.

Can two littermates share one carrier?

Sometimes. It depends on the airline, same as the age question.

Delta allows two pets of the same breed and size, between eight weeks and six months, to travel in one kennel if they fit and are compatible, and charges them as one pet. United doesn't: its in-cabin rule is one pet per kennel, with birds as the only exception. American's two-per-kennel rule is written into its checked pet guidelines rather than its carry-on section, so that's one to get confirmed rather than assumed.

Because the fee is charged per carrier rather than per animal, two siblings going to the same city can work out much cheaper per kitten on an airline that permits it. That's a real saving, but don't pass it on to a buyer until the airline and the aircraft are both confirmed.

What to do with this

• Don't treat eight weeks as universal. It's the floor on Delta and American. United won't take a kitten under sixteen weeks.
• Check the airline operating each leg, not the one on the ticket, and recheck after any schedule change.
• Confirm the destination state's requirement for your route before you book the vet.
• Size the carrier for the smallest aircraft on the itinerary.
• If two are going to the same city, confirm the airline's sharing rule before you quote a price that assumes it.