Socioeconomic status (SES) gradients may not be static across the lifespan, but instead may vary in strength across different life stages. This study examined the periods in childhood when SES and health relationships emerge and are strongest among US children. Data came from the National Health Interview Survey, 1994, a cross sectional, nationally representative sample of 33,911 US children ages 0-18. Parents were asked about family SES and child health status. Global health measures included overall ratings of child health, activity and school limitations. Acute conditions included childhood injuries and respiratory illnesses. For all global child health measures, lower family SES was associated with poorer child health in a gradient fashion (P < .001); these differences did not vary across age. For specific conditions, interaction effects of SES with age were found (P < .05). Interaction effects revealed that for injury and acute respiratory illness, expected SES gradients (lower SES with poorer outcomes) were evident during adolescence. In contrast, respiratory illness had a reverse SES gradient in early childhood. In sum, for global child health measures, associations of lower SES with poorer health throughout childhood suggest that factors that do not change with age (e.g., health care quality) may best explain overall health status. However, for acute conditions, the relationship between low SES and poor child health appears most consistently during adolescence. This suggests that normal development-related changes during adolescence, such as increasing peer group affiliation, may help explain these gradients. These patterns are important to understand for optimally timing interventions to reduce SES disparities in US children's health.