Human young have large heads and short legs. They move haltingly and with hesitation. Young people explore their bodies and practice movement in a way which we find disturbing in older people. Children play at what they see older people do, but as soon as what they do becomes mandatory, it stops being fun.
Elementary school children tend to move with exaggerated and overstated actions. Because they move so fast and so wildly, they have little control and often miss. This applies to their mind also, although the largeness of their mental actions can lead to great idealism if led even gently in the right direction.
Pre adolescents begin to focus their movements and have more purpose to their actions, this too applies to thought. By this time, most body proportions are as they will be in adulthood, all except for the head. Physical activity for the sake of movement itself becomes less important in recreation at this point, but still remains somewhat important in most people.
Adolescents tend to go back to large and somewhat sloppy actions and thoughts but not all do. Their hands and feet are large in proportion to the rest of their body and they have a growth spurt. Boys shoulders broaden, they get facial hair and their voices change. Girls get broader hips and a figure.
Adults legs are one and a half times the length of their torso. Their arms reach halfway down their thighs and their heads are one seventh of their height. Adults take accurate movement and thought past where pre adolescents have it.
We are accustomed to the patterns of young having large heads and exploring their bodies. Instinctively we think that things with large heads and eyes are cute because that is the most efficient way for our kids to look. When older people engage in young behaviors, we perceive something to be wrong.