Why must we grow old

Why don't we grow young!
Its so unnatural,
As we learn more, become more wise
have more experience on our side
we should be able to do more
rather than housing a brain that knows all,
in a body that can't see, can't hear
can't move, a body so fragile.
after becoming an adult,
if we could grow young with each passing year
with having better known the world
we could contribute so much
to the mankind
and to ourselves.
it's as if a human being
is no more than a fruit
either its not ripe
or it starts getting rotten
the time when it is at its best
is so scanty!