You've probably heard the adage that a dog's age in "human years" is 7 to 1 and you may expect a similar formula for cats. Not so. Aging and development are far more complex than this for cats ... and it doesn't really work for dogs, either.
In a Rush
The reason there isn't a simple formula for converting cat ages into equivalent human ages is that aging isn't a steady process. Your kitty will ratchet rocket-like from helpless babyhood to human "driving age" in under one year. Her first six months of life are relatively equivalent to slightly over 14 "human years" (that is, she'll be able to have babies of her own by six months, even though like a teenage human she'll still be emotionally and physically immature).
Here I Am!
Once cats reach between 1 and 4 years of age (depending on breed and adult weight) they finish growing and are officially adults. At this point, they have the next 10 years or so to cool their heels and enjoy being in their prime. It's as if all of a human's 20s through 50s pass between your cat's ages 1 to 4.
At around age 10 to 14, your kitty will enter her "senior years." What this actually means for her health and mobility, though, is entirely individual. Like with humans, the rate at which she actually ages (and declines) will be determined in part by her genetics, in part by her lifestyle while younger and in part by her ongoing diet and activity level and any underlying medical conditions.
Just like humans, some cats reach a "senior" age while they're still youthful and spry; others seem elderly and feeble before their time. Look at your cat rather than the calendar to determine whether her advancing years require more advanced veterinary attention.
Enough with these specifics -- is there a cheat sheet to convert "cat years" to "human years"? You bet! It's just slightly more complicated than you may have expected. A rough approximation is that a 1-year-old cat is equivalent to about an 18-year-old human. By age 2, your cat jumps to around 25, and then gains about the equivalent of four human years in age each additional year. This is by no means an exact science, and different sources use slightly different calculations, but expect your kitty to start needing senior care around 65 -- which is to say, age 12 or so.
Angela Libal began writing professionally in 2005. She has published several books, specializing in zoology and animal husbandry. Libal holds a degree in behavioral science: animal science from Moorpark College, a Bachelor of Arts from Sarah Lawrence College and is a graduate student in cryptozoology.