According to googling it, its 9-18 months, however ...
I've never adopted young cats before, but we went looking for one new cat aged 5+ and somehow got two of about 18 months. They're now coming up to 3 and one has filled out to a pleasant plumpness, while the other (who used to feel too delicate to me - they're both boys and picking up the fluffy guy just felt like nothing, felt like he weighed 8 pounds, versus the 18 pound bruisers I'm used to).
Well, this winter the fluffmeister has suddenly started seeming much more muscular and solid feeling.
Do they still grow at this age? If not in terms of their skeletal structure, then in muscle mass?