These sorts of decisions are the result of specific historic conflicts, and cannot really be understood without knowing about these conflicts. They aren't derived from first principles.

For instance, the US voting age was only lowered to 18 in 1971, partly in response to the Vietnam-War-era objection that 18-year-old men were being drafted to go kill or die overseas, but could not vote.

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.

I'd imagine history has a huge amount to do with this.

In Britain, the age of consent is 16. The age at which you can get married is also 16.

It's also illegal to sell alcohol to children under 18 (unless alongside a meal, with an adult accompanying them, in which case 16). It's not actually illegal to drink alcohol underage, and it's not uncommon in the slightest for parents to allow their kids to try their drinks at any age.

Britain is an old place though, so I guess things are this way because of centuries, possibly millennia of inertia.

The US is a young country, and as a result the laws are far most maliable - particularly by lobbyists. Puritans push the age limit of 'sinful' things up, capitalists keep the age limit of indulgences low.

I'm guessing it's because different parts of us mature at different rates. Brain development continues into the early twenties, so mind-altering substances are generally regulated with that in mind. Physical maturity ends in the late teens, so things like military service eligibility start there. Other activities are based on more abstract measures like responsibility, emotional stability, etc.

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.

The idea is that by those ages most people should be capable of doing those tasks responsibly. Not everyone is responsible enough to drive at 16, but most are.

To be fair, most first world countries seem to have settled on 18 for all of this. I think the wrong assumption is that the age limits are related to each-other, while they've been brought into law at different points in history and there hasn't been a need to change them. There are also the environmental factors, such as the driving age in the USA probably being lower as a consequence of a car-centric culture.