human subject child human subject https://nexus.od.nih.gov/all/2018/08/07/human-subjects-and-clinical-trial-glossary-updates/ A human subject classified as adult, which is defined by NIH as an individual under the age of 18 years. For the purposes of the NIH Policy and Guidelines on the Inclusion of Individuals Across the Lifespan as Participants in Research Involving Human Subjects, a child is defined as an individual under the age of 18 years. Oliver He