Fig. 1.

How age is counted differs between organisms. Different scientific communities use different methods for how the age of an organism is counted. This diagram shows the entire life of five organisms from fertilization to death, all scaled to the same length bar. Each bar shows in the red shade the time the organism develops before the “zero” time point when the age count typically starts. The blue shade marks the time before the organism is sexually mature that is included in the age count for each species. For humans (top), the 9 months of development before birth are excluded from the age count, but the approximately 12 years prior to sexual maturity are included in the average 77-year lifespan. Similarly, for mice (second from top), the approximately 20 days of development before birth are excluded from the age count, but the approximately 6 weeks prior to sexual maturity are included in the average 28-month lifespan. For Drosophila (middle), the 10 days of development (in the egg and as larva) prior to the eclosion of the adult animal from the pupa are excluded from the age count, while the 8 h prior to sexual maturity are included in the average 50-day lifespan. For C. elegans (second from bottom), the 2.5 days prior to the adult stage (2.5 h of development prior to egg laying, 12 h development to hatching, four larval stages) are excluded from the age count, but the 8 h prior to sexual maturity are included in the approximately 20-day lifespan. Finally, in the extreme example of periodic cicadas (bottom), the 4–6 weeks of development in the egg and the 17 years of development underground are excluded from the age count, but the 4–10 days before the adult cicada reaches sexual maturity are in the approximately 1-month lifespan