Recent studies project that temperature-related mortality will be the largest source of future damage from climate change, with particular concern for the elderly (whom it is believed bear the largest heat-related mortality risk) and humid heat extremes (which physiology suggests may have dire consequences for human health). Here, we study heat and mortality in Mexico, a country that exhibits a unique combination of universal mortality microdata and among the most extreme humid heat exposures. By combining detailed measurements of wet-bulb temperature with granular, age-specific outcome data, we find that younger people are particularly vulnerable to heat while older people are particularly vulnerable to cold: those under 35 years old account for 75% of recent heat-related deaths and 87% of heat-related lost life years while those 50 and older account for 96% of cold-related deaths and 80% of cold-related lost life years. We develop high-resolution projections of humid heat and associated outcomes holding historically observed exposure–response relationships constant. We find that climate change causes temperature-related mortality risk in Mexico to shift from older people (more vulnerable to cold) to younger people (more vulnerable to heat). As a result, under the end-of-century climate in the SSP 3-7.0 emissions scenario, temperature-related deaths among under-35-year-olds increase 32%, while deaths among other age groups decrease by 33%.