I think it depends to a degree on the cat, the breed & their past. Mine were 2 when I got them, but had a hard life before they were taken to the rescue. They've both definitely grown since then.
I know with horses the breed has a massive effect on growth. A Thoroughbred will be near enough fully mature at 2 years, whereas the heavier breeds tend to finish upwards growth at about 4 & can still be filling out for another year.
I've got the same question! My younger cat is about 14 months old and her body is really loooonnng (putting on a bit more bulk recently) but she still has quite a small head and kittenish face. Is that normal? She seems perfectly well but was the smallest of a rescue litter.