In many developing countries, the share of the population living in the adult ages is increasing. Despite these demographic shifts, there are still many gaps in the research on aging and adult health in developing countries. My first chapter uses data on Indonesia to study socioeconomic differences in adult mortality. I find that the size of socioeconomic differences is much smaller in Indonesia than in many HICs and not explained by behavioral risk factors. My results suggest that mortality inequality in middle-income countries may follow a trajectory that is distinct from the current and historical experiences of HICs. One surprising finding from my first chapter is that high blood pressure is very high in Indonesia and strongly predictive of mortality. My second chapter builds on these findings by examining the etiology of high blood pressure in Indonesia. Using fixed-effects panel data methods with 17 years of longitudinal data in Indonesia, I find that changes in weight are related to changes in blood pressure across the entire distribution of BMI. My findings reveal that changes in weight among lean individuals can still have consequences for blood pressure and that conventional risk factors for high blood pressure may not be sensitive indicators of disease in developing contexts. Underlying the entire study of individual aging is the question of why some individuals engage in behaviors that are known to negatively affect health. My third chapter uses data on U.S. twins to investigate the degree to which multiple adult health behaviors can be explained by a single set of characteristics. Our paper combines approaches from economics and behavioral genetics to determine the contribution of schooling, genetic endowments, and environments to unhealthy behaviors among U.S. adults. We find that most health-related behaviors in adulthood are largely idiosyncratic and likely not caused by single factors. The results from the three chapters suggest that greater attention needs to be given to context-specific determinants of behavior, health, and mortality. As countries around the world continue to age, understanding why differences in aging exist across and within populations can provide new insights to promote healthy aging globally.