I'm updating a talk on research issues associated with pediatric treatments. Before I start throwing around terms like "paediatric" and "child," I should take some time to note that these terms have some ambiguity in them. A child is typically defined as a person with an age less than 18 years. This is not a hard and fast definition, though, and certainly I would not quibble if you used 16 years or 21 years as the boundary between a child and an adult. There are certain activities, such as marrying, living on one's own, becoming a father or mother, that will sometimes change the status of a child in legal or social settings.
The age range 0 to 18 years is a very broad range, and it makes sense at times to limit the discussion to certain age subgroups. This again is an arbitrary choice, but the International Conference on Harmonisation offers the following categories for consideration.
- preterm newborn infants,
- term newborn infants (0 to 27 days),
- infants and toddlers (28 days to 23 months),
- children (2 to 11 years), and
- adolescents (12 years to adult).
(Source: ICH Topic E11: Clinical Investigation of Medicinal Products in the Paediatric Population, www.emea.eu.int/pdfs/human/ich/271199en.pdf).
Other definitions of child and other categorizations of age groups may certainly be appropriate. A lot depends on the context of the particular research problem.