Early-life exposure to ambient air pollution has been associated with adverse birth outcomes and poorer health across the life course. The mechanisms underlying these associations are still unclear, but differential DNA methylation might be involved. Nitrogen dioxide (NO2) and particulate matter (PM) exposure in pregnancy have been found to be associated with newborn DNA methylation. The objective of the present study is to examine the associations of air pollutant exposure during early life with epigenetic age acceleration at birth, in childhood and in adolescence in a multi-cohort setting.