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Objectives Chronic conditions and multimorbidity are increasing worldwide. Yet, understanding longitudinal changes in multimorbidity over the lifecourse is limited. We compared the age-trajectory of multimorbidity by birth cohort and examined effects of socio-demographic (e.g. sex, income) and behavioral risk (e.g. obesity) factors in multimorbidity. Methods Using data from the Canadian Longitudinal National Population Health Survey (1994–2010), we examined 10,186 participants born 1925–1974 grouped in 5 birth cohorts. Data on the number of chronic conditions (up to 17), income, education, and behavioral risk factors were collected biannually. We used multilevel logistic growth modeling techniques for analyses of multimorbidity defined as the presence of 2+ chronic conditions versus 1 or none. Results We found significant cohort differences in the age-trajectory of multimorbidity: at corresponding ages, each succeeding recent cohort had higher odds of reporting multimorbidity than their predecessors. Access to healthcare did not fully explain these differences. Women, having lower income, being obese, smoking, and a sedentary lifestyle had increased odds of reporting multimorbidity. Obese individuals, particularly Gen Xers and younger boomers, reported multimorbidity at an earlier age than those of normal weight. We observed divergent trajectories of greater multimorbidity for lower than higher income individuals. Furthermore, after accounting for age and risk factors there was an apparent increase in reporting multimorbidity over time (period effect) across all ages. Discussion Our findings indicate that multimorbidity is not only becoming the norm, but is emerging earlier in the lifecourse and particularly for low income and obese individuals from more recent cohorts. The findings point to the importance of planning interventions and policies to deal with more recent birth cohorts entering into older age with worse health than their predecessors. [ABSTRACT FROM AUTHOR]