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
It’s all arbitrary and most of it is based on nothing but cultural assumptions. Almost everything people think they know about teenagers, even the existence of “teenagers” as a group, can be traced to culture or marketing far more concretely than anything inherent or biological.
As others have said, there are no solid principles backing practically any of this. It’s all junk science and looking for confirmation of assumptions.