Some felines can become sexually mature at 4 months....so if you want to get techinal - even at that young age they can in certain situations be considered sexually prime (Which is why the old school train of thinking where vets make you wait until 6 months to have them altared is not good).
But as far as being full grown goes- 8 months to a year- buy this time they have all adult teeth and for the most part have finished growing. They finish developmentally "growing" around age two. Between kittenhood and two years of age you'll see them go through different changes and phases. Some breeds like Maine Coons and Turkish Angoras don't finish developing fully for several years when it comes to size/coat.
Does that help?
(Developmentally speaking- I keep all of my foster kittens/etc on a high quality kitten chow for an entire year to help enhance development. After having tons in and out of my house i've found that works best in my opinion.)