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.