Bone age is a measure of the degree of skeletal maturity of a child, i.e. how far the child has advanced in its development of the skeleton. Conventionally this is determined from an X-ray image of the hand.
Skeletal maturation is controlled by hormones, and the same hormones also govern the timing of puberty, so a child with delayed skeletal maturity is also likely to have puberty late, e.g. late menarche.
Bone age is measured in years, most often using the Greulich-Pyle bone age scale. If a child has bone age 10 years it means that the child maturation is as advanced as the average of the 10-year old children from Ohio in 1930-1940 that were studied by Greulich and Pyle.
Different population groups mature at different speeds. For instance, healthy children mature faster than children living under poor conditions. Present-day Western-European children mature approx 2-4 months later than the very healthy children studied by Greulich and Pyle, so the average bone age of 10-year-old children is approx. 9.7 years.
The standard deviation of bone age at a given age is approx 1 year, which implies that in a random group of 7 boys of the same age, there is on average 3 years difference between the most and the least mature boy, i.e. the most advance boy has puberty 3 years before the least advanced.
The growth spurt of boys occurs on average at 13.5 years of bone age, and the girls have menarche at bone age 13.2 years, on average.
The following movie shows a series of images from the first Zurich Longitudinal Study (courtesy of Zurich Univ) show annual X-rays of the same bone. The images were aligned to each other by Nicolai and Laus as part of their Master's thesis at the Technical University of Denmark.
Maturation of the third metacarpal
Maturation of the third proximal phalanx