EPA views childhood as a sequence of lifestages, rather than considering children as a subpopulation.

The use of the term "subpopulation" is ingrained into both EPA's past practice as well as various laws that EPA administers such as the Safe Drinking Water Act Amendments (1996). Prior to the publication of the 2005 Guidelines for Carcinogen Risk Assessment and the 2005 Guidance on Selecting Age Groups for Monitoring and Assessing Childhood Exposures to Environmental Contaminants, the EPA described all types of groups of individuals as "subpopulations." Both of the 2005 guidelines, above, recognized the importance of distinguishing between population groups that form a relatively fixed portion of the population (e.g., groups based on ethnicity) and lifestages or age groups that are inclusive of the entire population. The term "lifestage" refers to a distinguishable time frame in an individual's life characterized by unique and relatively stable behavioral and/or physiological characteristics that are associated with development and growth. Thus, childhood should be viewed as a sequence of lifestages, from birth, through infancy and adolescence. The later guidance recommends the use of the following postnatal childhood age groups:

• Age groups less than 12 months old include: birth to <1 month, 1 to <3 months, 3 to <6 months, and 6 to <12 months.
• Age groups greater than 12 months old include: 1 to <2 years, 2 to <3 years, 3 to <6 years, 6 to <11 years, 11 to <16 years, and 16 to <21 years.

There are other lifestages that may be important to consider when assessing human exposure and risk including: pregnancy; nursing; and old age.​

When assessing early life risks, consideration is given to risks resulting from fetal exposure via direct exposure to the pregnant mother, as well as risks due to postnatal exposures.