I thought it was about the test where children reach with their right hand over their head to touch the left ear - a common tongue-in-cheek test for first grade

I thought it was about the test where children reach with their right hand over their head to touch the left ear - a common tongue-in-cheek test for first grade readiness. Apparently the test has some real foundation. That the proportion of the limbs (and relative size of the head) only allows children to touch the ear at about 5 years old.