For instance: age of sexual consent, age for legally drinking alcohol, age for driving, age for voting, age for participating in pornography
Depending on the place, each of those requires a different minimum age. Why is that? Are some activities “more adult” than others? Using USA as an example: legal drinking age is 21, legal driving age is 16, age of consent varies between 16-18.
Not asking about different countries/states having different ages, but any single place having different ages for different adult activities
Yep. But also, fake first-principles explanations are commonly offered when people ask “Why is the rule thus-and-so?”
“Why can’t I vote yet? (or: marry, buy a gun, etc.)?”
“You’re not old enough.”
“Why not?”
“Because 18-year-olds are just more mature than 15-year-olds.”
“Why? Who says?”
“Um. The Constitution!”
“But it used to say something different.”
“Yep. We know better now.”
“Says who?”
“Democracy!”
The 15-year-old correctly assesses that these are not real explanations, but rather rationalizations of a rule that was decided not from first principles, but rather through people in history arguing over it, sometimes protesting or even fighting, and changing the rules.