The musculature, dress, expressions, and mannerisms are all adult. Consider how silly this well received argument was. The same artists had ample skill to paint adults, but they couldn't paint kids. Aries suggests another explanation, the one generally accepted today, namely that they couldn't paint young people as children because they were not children. In their cultures they were little adults, and this is precisely what the artists saw.
Childhood is a later historical creation.
On Aries' view, once the institution of childhood began to emerge the situation of the young person began to change in society. First they were named children. A theory of innocence of the child emerged. Children were to be protected from adult reality. The facts of birth, death, sex, tragedy, world events were hidden from the child.