Why is each lifespan not clearly defined by age?

Lifespans are not clearly defined by age because individual aging processes are influenced by a combination of genetics, lifestyle, environment, and access to healthcare. Additionally, different species, populations, and even individuals within the same species can experience varying rates of aging and health outcomes. Life expectancy statistics often reflect averages that may not apply universally to every individual. As a result, defining lifespan strictly by age overlooks the complexities of biological and social factors that contribute to aging.