Milestones Of Development

The list for physical development (Table 1) includes a number of familiar milestones for infants, most related to gaining control over their bodies so that they are able to move about on their own. The list of milestones for the preschool years will also be familiar. It is during this time that children become toilet trained and learn to use simple tools, such as forks and spoons. The first permanent teeth erupt around six years of age. Although children across the globe are typically weaned by age three or four, they are not able to eat an adult-style diet until they have most of their permanent teeth. This means that adults must specially prepare food for children years after they have stopped nursing. This is a pattern seen in no other animal and makes the period of "childhood" unique to the human species.

Physical growth is slow and gradual between the ages of about six and eleven, when the adolescent growth spurt begins (sometimes a bit earlier for girls). The rapid growth at this time, which occurs later for boys, coupled with the development of secondary sexual characteristics, marks the physical transition to adulthood. Girls' first menstrual period (menarche) usually occurs about two years after the onset of secondary sexual characteristics, and both boys and girls have a period of relative infertility, lasting several years, after they have become sexually mature. Although the pattern of puberty described here is universal, the average age at which girls reach puberty has been decreasing over the past two centuries, primarily because of better nutrition and health. There is also evidence that girls from high-stress, father-absent homes reach puberty earlier than girls from low-stress, father-present homes, reflecting the role that social factors can have on physical development.