All these people talking about underage pregnancy in the comments... Japanese highschool includes 18yo students, and don't forget that some countries do have the age of consent below 18. In the UK it's 16, and in Japan the national age is way too low but the individual regions can and do set their own, usually between 16 and 18 iirc.
Remember: USA hasn't quite conquered the world yet, so there are still places where having sex and drinking alcohol aren't deemed more "adult" than owning a gun or joining the military.