Physical Growth

Different body structures and functions often mature at differing rates, and they achieve adult status at different average chronological ages. For example, the three tiny bones of the inner ear (the incus, malleus, and stapes) are mature before birth, while the last bone to achieve adult status (the clavicle or collarbone) does not do so until approximately twenty-five years of age.

Even within groups of healthy children, there is considerable variation in the timing of the same maturational processes and events. For example, the first menstrual period of girls, or menarche, signals achievement of one aspect of adult reproductive functioning and is a widely used maturational indicator. (The corresponding but less noticeable event in boys is the first production of sperm cells, or spermarche.) The average age at menarche for girls in the United States is approximately 12.8 years of age. About two-thirds of U.S. girls will attain menarche within one year of the average timing, and about 98 percent of all girls within two years. For healthy girls, this variation in the timing of menarche is due to inherited patterns from their parents. Age at menarche (and most other maturational timing) can be delayed by malnutrition and infectious disease, and less commonly by hormonal dysfunction.

The chronological age at which maturational events occur provides a measure of the relative timing of that event in the child's growth and development. In addition to menarche, other examples of maturational events whose timing may be of interest include onset of ossification of bony centers (visible in X rays), eruption of teeth, first walking, first appearance of pubic hair, the age when the adolescent spurt is at its peak velocity, and the final fusion of the growing centers of long bones.

Of course, these maturational events are really biological processes that occur progressively in the developing child and the "event" is really just an arbitrary point in the developmental process that has been defined by auxologists so that it can be measured more easily. Some maturational processes have been more or less arbitrarily defined in stages or grades so that the progress through the stages can be measured. The progressive development of the secondary sexual characteristics associated with sexual maturation is a common example where such stages have been applied. The development of breasts in girls, penis and scrotum in boys, and pubic hair in both genders have carefully described stages of development that pediatricians and endocrinologists use clinically and that are also used by researchers who are interested in normal and abnormal adolescent growth and maturation.