On average, a girl becomes heavier than a boy around the age of nine or ten. This is a temporary state, however.
Growth and Weight Differences Between Boys and Girls
While boys and girls have similar weights at birth, and are roughly equal in weight around age eight, a typical girl surpasses a typical boy in weight around age nine or ten. This difference is maintained until approximately age 14.5. [Reference: At age 14 she is surpassed again in height by the typical boy, whose adolescent spurt has now started, while hers is nearly finished. In the same way, the typical girl weighs a little less than the boy at birth, equals him at age eight, becomes heavier at age nine or 10, and remains so until about age 141/2.]
This weight difference is largely due to the timing of puberty. Girls typically experience their growth spurt earlier than boys. After approximately 14.5 years of age, boys generally surpass girls in weight as their adolescent growth spurt continues.
Note on Individual Variation
It's crucial to remember that these are average trends. Individual growth and weight vary considerably depending on genetics, nutrition, and other factors. The provided references highlight weight differences between individual boys and girls of the same age as well, reinforcing the significance of individual variations in weight. [Reference: I have two sibling Ragdolls, my boy is 5 kg (11 lbs) and my girl 4.5 kg (10 lbs). They are now 1.5 years old! I wonder if they'll still gain ...]
