So sad

In Australia, you finish high school at 18 usually. But I started a year early so I finished school at 17

In America, the average high school graduate is 18 years old when they graduate. Recently more US states have raised the minimum starting age for school to 5 years old- and traditionally there was a break point based on the month you were born. So at a school where the minimum age is 5, and school starts in September- a child who would turn 5 in November wouldn’t be able to enroll that year. They would turn 5, then the following September would enter school with kids who were born a year later than them. So that child would turn 6 while in kindergarten while the rest of their class would be 6 right after the start of the school year and many of their peers wouldn’t turn 6 until near the end or after the end of the school year- making them an increment of one year older than their class average and sharing a both year with students a grade higher than them.

Now, even without doing programs or taking extra work to graduate early, many Americans graduate at 16 or 17. It mostly depends on how early they started school (which depends on where they live and the rules of that particular school system on minimum age etc), and some schools combine grades or do other things that can change this. Also please note- not all schools require a child be “X age before the start of school.” And those with an age requirement often have a date OTHER than the start of school. For a 2021 school year they might say “child must turn 5 (or 6 or 4 etc) before 8/23 (or 2/21, or whatever date they choose.)” so when you are born, where you live, and when your parents enroll you are the biggest factors.