Why is it difficult to compare the life spans of animals with very different stages at birth?

Comparing the life spans of different animals is difficult because "birth" is an inconsistent starting point across species. While birth is generally considered the beginning of an organism's life span, the developmental stage at which this occurs varies significantly.1

For example, a newborn mammal like a deer may be capable of walking within minutes, whereas a kangaroo is born in a highly underdeveloped state and requires further growth in a pouch. Similarly, many marine invertebrates hatch as larvae consisting of only a few cells, which is not comparable to the more advanced state of other newborns. Because these organisms are at vastly different stages of maturity when they are considered "born," using birth as a universal marker for the start of a life span is a poor criterion. To make accurate comparisons, these developmental variations must be accounted for.1