Do you mean in height, or in other departments?
Most people reach their adult height by 18, but I think can continue to grow a little afterwards. I'm half an inch taller at 45 than I was at 18 or 20, and half a shoe-size larger (13...).
Depends partly upon the age of puberty, which in girls can be anything from 9 or 10 to 20 or so. Most people of either sex grow taller faster about the time of puberty, but women also grow larger hips as well as more obvious bits. Men grow more muscles and their bones become more angular, among other things.