Family socioeconomic status (SES) is an important source of child health disparities in the USA. Chronic stress is one way SES may impact children's physiology with implications for later health inequalities. These processes may work differently across childhood due to differences in exposure and susceptibility to stressors at different ages. We assess associations between family SES and one biomarker of chronic stress exposure-low-grade inflammation detected by elevated C reactive protein (CRP)-and evaluate differences in the associations by child age.