grade + 5 or 6 is a quick conversion from grade to age in the US, since most people start grade 0 (kindergarten) at 5 and end at 6. So someone in 11th grade typically started the school year at 16 and then ended the school year at 17.
In my state (California), and I believe similar to many other states, any child who is 6 must be enrolled in school, so pretty much the oldest you’ll see is a kid who turned 6 the summer right before kindergarten, which means turning 19 in the summer after high school. Students must be 5 before September 1 to be enrolled in kindergarten, so the youngest you’ll typically see now is someone who turns 17 right at the start of 12th grade.
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