A widely held belief for determining whether a cat is middle-aged or old is that one year in a cat's life equals four in a human's. In truth, the situation is not that neat, and if you think about it, you can easily see why. Under a "one equals four" rule, a 1-year-old cat would be the equivalent in terms of mental and physical maturity to a human 4-year-old, and that's clearly off.

A better equation is to count the first year of a cat's life as being comparable to the time a human reaches the early stages of adulthood — the age of 15 or so. Like a human adolescent, a 1-year-old cat looks fairly grown up and is physically capable of becoming a parent but lacks emotional maturity.

The second year of a cat's life picks up some of that maturity and takes a cat to the first stages of full adulthood in humans — a 2-year-old cat is roughly equivalent to a person in the mid-20s.

From there, the "four equals one" rule works pretty well. A cat of 3 is still young, comparable to a person of 29. A 6-year-old cat, similar to a 41-year-old person, is in the throes of middle age; a 12-year-old cat, similar to a 65-year-old person, has earned the right to slow down a little. A cat who lives to be 20 is the feline equivalent of nearly 100 in terms of human life span!