How long does it take for a kitten to grow?

They are sexually mature as soon as six months under adverse conditions, such as starvation, where their body tries to get a new generation going before it's too late, or among many Oriental breeds who mature early.

Physically, full size cats can take four or five years in such large breeds as the Maine Coon, Norwegian Forest Cat, and Siberian. A cat under adverse conditions can actually postpone their full growth until times get better. That ten month old you adopted that the vet confidently predicted wouldn't get much bigger? They can double their size in the next year, with a good home, consistent food, and knowing they are loved.

Under ideal conditions, a cat, depending on breed, should be done by the age of two years; first year, gets their bone structure done, second year, filling out their muscle structure. Neutering or spaying has no effect on their eventual size, though male cats, done before the thickening of the neck and broadening of the cheekbones under the influence of male hormone, will not develop these traits.