These graphs were created by dividing the girls growth chart by the boys growth chart. Lower percentiles (blue lines) represent smaller (shorter/lighter) people, while the highest percentiles correspond to larger people. The yellow line represents the typical person (50th percentile = median). Note that the y-axis scales are different for the two graphs; weight ratios vary more significantly than height ratios.

The graphs shows that girl and boy babies are born at close to the same size, but boys get larger more quickly in the first six months of growth. The ratios then increase, approaching one as girls catch up in size. Around the ages of 7-9, the typical girl is about the same size as the typical boy. Then girls hit puberty first, and from 10-13 they are generally larger than boys. Buy once boys hit puberty, they catch up quickly, and become taller and heavier after the age of 13.

Data source: http://www.cdc.gov/growthcharts/