Early childhood, defined as the period from birth to eight years old, is a time of remarkable growth with brain development at its peak. During this stage, children are highly influenced by the environment and the people that surround them. Early childhood comprises a number of life stages, marked by developmental milestones. Here, we define early childhood as the period from birth to age eight, although we also recognize the importance of quality prenatal care in early childhood outcomes. Ours is a definition shared by many leading national and international organizations. We end with age eight because it reflects an important developmental milestone. Age eight corresponds to third grade, a critical year for mastery of the reading skills upon which further learning will build and a reliable predictor for future education success.

Although ages zero to eight represent a clear developmental continuum, there are good reasons why funders and programs specialize in one stage. The needs of children and their families evolve and different organizations are positioned to play different roles at different stages. For example, infants and their parents may be more easily reached through the health system, while the prospects of older children may be more easily influenced through their elementary school. Ideally, investments across all stages would be sequenced and coordinated so that the child who benefits as an infant from one program moves seamlessly to environments that will continue to support their development.