A 1 year old cat is equal to 15 human years.
So if you have a 1 year old cat the 1 year old cat has the developmental ability of a 15 year old human would have.
A 2 year old kitten has the developmental ability of a 25 year old human and is also 25 years old in human years.
After 2 years of age to calculate the age of a cat in Human years you equate each cat year to 4 years in human years.