Kittens grow fast! By the time they are 6-8 weeks old, they are at the developmental equivalent of an 18-month child. By Kitty's first birthday he is at the same stage as a 15-year old youth. At the age of 2 years a cat is considered to be equivalent to a 24-year old person. Each successive year is deemed to be equal to 4 human years. On the average the cat lives to be 12 to 15 years, but 20 years (96 human years) is not uncommon.