What defines the concept of lifespan?

The concept of lifespan is best defined by the greatest age reached by any member of a population. This definition encapsulates the upper limit of life expectancy for that group, emphasizing the potential maximum longevity an individual can achieve. Lifespan is a measurable biological fact, often seen in studies of various species to determine their maximum age under optimal conditions.

While the other options reference aspects related to age and population, they do not capture the essence of lifespan accurately. For example, the average age reached by members of a population refers more to life expectancy rather than lifespan. The age at which individuals begin to age addresses a particular stage in development but does not reflect the maximum possible age. Similarly, the predicted duration of life for a species may relate to life expectancy forecasts and demographic studies but does not denote the concept of lifespan as the maximum age reached by individual members. Thus, the correct understanding lies in recognizing lifespan as the longest age an individual of a population has attained.