During early childhood, children ages 4 through 8 or 9 years (the latter depending on the onset of puberty in each gender) undergo major changes in growth rate and endocrine status. For many nutrients, a reasonable number of data have been available on nutrient intake, and various criteria for adequacy serve as the basis for nutrient reference values for this group. For nutrients that lack data on the requirements of children in this age group, the nutrient reference values must be based on extrapolations from other life stage groups.

The adolescent years are divided into two categories. Several conclusions support the biological appropriateness of creating two adolescent age groups within the DRI framework (IOM, 2006):

• The mean age of onset of breast development for white girls in North America is 10 years; this is a physical marker for the beginning of increased estrogen secretion (in African American girls, onset is about a year earlier, for unknown reasons).

• The female growth spurt begins before the onset of breast development, thereby supporting the grouping of 9 through 13 years.

• The mean age of onset of testicular development in boys is 10.5 through 11 years.

• The male growth spurt begins 2 years after the start of testicular development, thereby supporting the grouping of 14 through 18 years.

Adulthood was divided into two age groups, in part due to consumption of higher nutrient intakes during early adulthood compared with later in life. Mean energy expenditure decreases from ages 19 through 50 years, and nutrient needs related to energy metabolism may also decrease (IOM, 2006).

The age period of 51 through 70 years spans active work years for most adults. After age 70, people of the same age increasingly display different