I’m wary of simplification; the complex cannot be reduced to the simple without removing an element of truth. Mark Manson's The Four Stages of Life presents neat ideas.
Each stage represents a reshuffling of one’s life priorities. It’s for this reason that when one transitions from one stage to another, one will often experience a fallout in one’s friendships and relationships. If you were Stage Two and all of your friends were Stage Two, and suddenly you settle down, commit and get to work on Stage Three, yet your friends are still Stage Two, there will be a fundamental disconnect between your values and theirs that will be difficult to overcome.
Stages are a useful and easy tool for breaking down the complex into the simple. The desire for taxonomy is strong, and the need for validation through checkmarks against a list is equally powerful. But I can't help but think that assessing and measuring your life through an external framework is dangerous and misguided.
I'm loathe to paraphrase Richard Dawkins on the internet, he's right about there being no "first human". Creating stages means an artifical simplification of existing conditions. This reductionist thought means that everything appears more simple than it actually is.
The idea that you can make life understandable, and your place within it, is nothing but a seductive fallacy.