I once came across a statement suggesting that the ancient Chinese believed that life unfolded in stages of length 7 years (the phrase "ancient Chinese" alone begs the question, since China is a great and vast nation with many different peoples...). In my experience, their observation seems to hold true, and furthermore, when I try to break down periods of life into stages of length 5 or 6 or 8 or 9, it doesn't seem to hold. I think they were onto something biological with the "7 year" period, and here's a brief break-down, extrapolating from my own life and observations of others:

Thinking back in my own life, around the time I turned 7, I started building solid friendships at school, friendships that didn't involved my family, so in a way that was when I began to move out into the world on my own.

Around 14, maybe a bit younger for women, everyone experiences sex changes that have profound physical and mental ramifications.

Around 21, again maybe a bit earlier or later, most people begin to move out into the world, become fully self-supporting, establish homes of their own, start having serious relationships/partnerships with a sexual element, and perhaps start feeling the very earliest effects of aging (when I was around 21 I noticed I started having to stretch before jogging, whereas earlier in life this was largely unnecessary).

Around 28, the biological mechanisms which drive people to make babies really come into full effect, especially with females. I've noticed this over and over with female friends: once they hit the age of 28 or so, if their energy and attention isn't already occupied with a family, that go into "baby-crazy" mode. I think males experience something similar, but haven't noticed it clustered around a specific age as with females, perhaps since males always have seed available, whereas females can't reproduce past a certain age.

There is a saying to the effect that "At 35, the mind begins to come together, just as the body begins to fall apart".

Does anyone on here find this model for life development corresponds with their personal development or what they've observed in other people? Perhaps members from non-Western cultures (my observations derive from growing up in North American culture) or members who have experience with ages past 35 (42, 49, etc) can provide further extrapolations as well.