The rich live longer than the poor. How and when these differences arise over the course of life is not well understood. Using rich administrative data from the Netherlands, we construct an index linking chronic disease profiles to mortality risk at old-age. Using our index we demonstrate that health inequality arises much earlier in life; by age 35, the bottom half of the income distribution has the same disease burden as those aged 50 in the top half. Approximately 60% of the divergence across income groups is due to low-income individuals developing chronic illness at a faster rate, rather than chronically ill individuals sorting into lower-income groups. Using linked health survey data, we then examine the contributions of mediators to the incidence of chronic diseases over the life-cycle. Socioeconomic and geographic factors explain most of the variation, while individual health behaviors play a more moderate role relative to prior literature.