I’m not asking this from a biology lens, I get that aspect, but more from the sociology/anthropology lens. My question is which culture started using 18 as this hard cutoff, when, and under what reasoning.
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I think it’s primarily about the **rate of change** (derivative) w.r.t. time.
Most people have completed the time in their lives where their outward physical appearance (including height) changes most rapidly. The rate of change slows down by to 1/1000th (or something like that) of their peak childhood rate for the rest of their adult lives. For instance,
similarity(age_18, age_40) >> similarity(age_10, age_18)
Physical appearance often dictates our first impression of a person’s character. Age is often a good predictor of behavior, so we use that to help us bin them into a particular category.
Ages <18 are usually pretty easy to pin-point within a guess of +/- 2 years. On the other hand, a person that looks 30 +/- 10 could very well be anywhere between 20 and 40. Since it’s much more difficult to visually determine the age for >18 year olds, we throw them into the same broad category of “non-senior adult”.
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Disclaimer: This is just my own armchair redditor theory.
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