This difference is present by age 4 and the gap gets larger. This difference is quite evident by age 2 and gets larger.
Boys are stronger in arm and upper body strength.
From an ethological view, this indicates that upper body strength was important for the survival of males. This trend continues and even in adulthood men avert gaze more while women maintain eye contact more. This sex difference emerges around age 3. Note Sioux game. This pattern has been found in US, Japan, Bali, Kenya, India.