Childhood (youth is a modern invention), Maturity, Middle age, Elderhood - then death...
The stages of life make it possible for humans to live in earth:
to become human (by the childhood stage, with its development and education)
to be productive and reproductive (in the middle two stages)
to become less worldly and more spiritual (in the second half of life).
Death makes salvation possible - without death a creature like a human and with free will would be damned, sooner or later; and to have free will is intrinsic to humanness.
Imagine: who could remain uncorrupted if they were a perpetual modern teenager, subject to the impulses and temptations of the typical modern teenager, for eternity?
Just as well we age, just as well we die.