Many cultures over thousands of years – and here I won’t back this assertion up. Just assume it’s true for now. Maybe I’ll toss references some other day – have a theory about “stages of life.” These can range from one monolithic life of whatever length to two stages (childhood and then adult) to four or five or more...like a play in "n" acts. My favorite framework, for reasons all my own, is the Hindu/Vedic system of 4 life stages which feels about right to me even though advances in tech and medicine might nudge the boundaries and definitions a bit. It is time tested in the sense that it has been around for a good long while. Here, stripped directly from Wikipedia, is how the stages are described in that ashrama framework.