ESA letters do not technically expire, but its highly recommended to renew them annually. That’s because the healthcare provider who wrote the ESA letter will not be able to validate a letter unless there is a current evaluation on file. It’s especially important to have a recent letter if you are moving or planning to move. Please see this article for more details: https://esadoctors.com/do-esa-letters-expire-how-to-renew/

Your cats can absolutely be emotional support animals! Cats and dogs are in fact the most common emotional support animals. Cats are a wonderful source of comfort for people with conditions like depression, anxiety or PTSD. They can be both a calming and uplifting presence. For more information about cats as emotional support animals, please see this link: https://esadoctors.com/can-cats-emotional-support-animals/

They cannot deny you housing solely because of your need for an emotional support animal. We recommend being upfront with landlords about your ESA needs.

Could they then deny my application if I do? Or should I apply then disclose once they approve my application?

You will have to disclose that you are requesting accommodation for an ESA some time before your ESA moves into the apartment.

Unfortunately, ESAs are generally no longer accepted on flights. Only service dogs can board flights under the DOT’s new rules for air travel in the US: https://esadoctors.com/how-to-fly-with-a-dog-travel-guide/