The official average lifespan of a cat in the US is 13-14 years old at this time. However, that includes cats who go outdoors, as well as those who live completely indoors. There are no official statistics on cats who live completely indoors (i.e., a house cat) that I am aware of.
In recent years, we're seeing more and more cats who live beyond 15. And many vet practices report several cats as patients who are in their 20s. While it's certainly not common for cats to live into their 20s, it does seem to be happening more often. I attribute this to better veterinary care and better awareness of owners. The internet has helped a lot because owners now ask questions, join groups, read information online and are far better informed about what to feed, and what treatments can help their cats' illnesses.
One of the vets I know has seen a cat as old as 26. I know someone who has a 27 year old cat. The oldest reported in history was 32, but it wasn't conclusively proven.
My oldest cat is 19.5, but I have several friends with cats that are 20-23.
A lot depends on genetics, but it's generally very possible with good care for a cat to live into its mid to late teens and much much more.
Good luck.
