The assumption that mosquito survival remains constant and that it is independent of age was tested with free-ranging Aedes aegypti (L.) (Diptera: Culicidae) by using a “synthetic cohort” mark–release–recapture method. Mosquito age cohorts (1–2, 3–4, and 19–20 d) were released simultaneously in homes in rural Thai villages during dry and rainy seasons (2002). Significant age-dependent effects were detected during the dry and rainy seasons. More young than older mosquitoes were recaptured (1–4 versus 5–20 d). The best predictive fit for recaptures was obtained using a quadratic function of age. Our results provide the first field-based evidence for age-dependent Ae. aegypti mortality in which the death rate increases with advancing age and highlight the need for research on ecological and epidemiological aspects of this process.