I think the answer depends on what part you are referring to- the sexual maturity, the social maturity, or the physical maturity. I thought cats reached sexual maturity earlier...like at 6-7 months, then, socially, I would guess closer to one year. As to physical maturity, I think that is closer to two years. My 'kittens' are 2 years 4 months. They are totally all grown up! Finally! But, they still act like the kittens of the bunch.