Why is the birth of a kangaroo not directly comparable to the birth of a deer when measuring life span?

The birth of a kangaroo is not directly comparable to that of a deer when measuring life span because of fundamental differences in their reproductive strategies and developmental stages.

Deer are placental mammals, meaning their young undergo a relatively long period of development within the womb before birth. In contrast, kangaroos are marsupials, a group characterized by premature birth. A kangaroo is born in a highly vulnerable, embryonic state—often only about 2 cm long—and must immediately crawl to the mother's pouch to continue its development while attached to a teat for weeks or months.123

Because the "birth" of a kangaroo represents only the beginning of an extended period of external development, it does not mark the same developmental milestone as the birth of a placental mammal like a deer. Consequently, using birth as a standardized starting point for measuring life span or developmental maturity across these two groups is biologically inconsistent, as the kangaroo's early life is spent in a state of continued maturation that occurs inside the womb for the deer.1