Quick Answer: In your early 20's. To answer this question more precisely we have to distinguish between the brain part of the skull and the face. A child is born with a brain about 25% of its ultimate adult size. By age 7 or 8 years, it is 95% of adult size and is finished in most children before puberty. The face grows more slowly, so all the skull bones are not finished growing until the early 20's with the eruption of the wisdom teeth. The difference in growth pattern is typical of most mammals such that young animals have relatively small faces and look cute to us.