How many human years is a 15 year old cat?

Could you elaborate on the correlation between a cat's age and its equivalent in human years, specifically for a 15-year-old cat? I've heard that cats age differently than humans, and I'm curious to know what that means in terms of their lifespan and maturity. Is there a general rule or formula to determine this? Also, what are some of the behavioral changes or physical characteristics that might indicate the maturity level of a 15-year-old cat? Your insights would be greatly appreciated.