Is aging and relatively short lifespans a physiological limitation, or actually selected for by evolution?

The primary mechanism for evolution is birth, followed closely after by death.
Animals are much more energy intensive compared to plants, which don't particularly see the effects of aging, so they need to compete more.

I imagine that evolution selects for shorter lifespans in animals to encourage new births and to encourage the ``retirement'' of older animals. The resultant increase in adaptability ensures the continuation of the species.

How am I doing so far?