In Britain, you are considered an adult when you turn 18. In America, that's true for only some things, like voting and signing contracts. You can drive at 16 (or younger in some states), but you can't buy alcohol or tobacco until you are 21, and some companies will not rent a car to anyone under 25. I once explained to a foreign visitor that Americans let their kids have jobs, date, and drive in high school to make sure a parent is available to help them learn those skills. The alcohol and tobacco laws were a response from the government to too many people dying.
Laurence Brown, who celebrates his birthday on Christmas Eve, also covers the difference between other adult things like traveling, taxes, home ownership, and hosting holiday parties in this video about how the United States is an outlier in transitioning to whatever passes for an adult life. There's a 75-second skippable ad at 1:55.
