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
In addition to vague cultural expectations, the country’s surrounding legalities, daily necessities and safety nets play a role.
E.g. in Norway, we have functional public transport and easy access by foot or bike to shops. Also our roads are - by standards of countries not consisting of fjords separated by mountains - extremely narrow, winding, and wildly varying. So we both need some more maturity in drivers, and people well into adult age can and do manage perfectly fine without cars. Our age limit is 18.
In the US, the roads are generally wider and even, buses “are for poor people”, plus there are places you can’t realistically even get to without a car, so it is both safer and more necessary with an age limit of 16.
Similarly for age of consent vs legalities around abortion, ease of access to prevention, and sexual education. In countries with good sex ed and unproblematic access to prevention, the age of consent tends to work well when lower.