After graduating high school at age 18 today, the students go on to a four-year college. Age 22 is correct. Age 23 if they're in a five-year program like a cooperative or an internship program.
Today, in most school districts, there's a very early birthday deadline that holds kids back a grade. In my district, a child cannot start Preschool unless he turns four by Oct. 1.
Both my oldest child and I have November birthdays. When I was a kid, I was allowed to start Kindergarten at age four turning five. He wouldn't have been allowed to start until he was five turning six. So, I graduated high school at age 17; he would graduate at age 18 if he were in school.
I have another child who misses the deadline by only nine days. And the school is strict about that deadline, too. Even if the school were more flexible, most parents adhere to the deadline, so your kid would be the youngest in his class. Maybe he can do the work, but will he socialize well with the older kids?
In some districts, they're moving the birthday deadline back into August!
I guess that makes sense. I've never memorized that birthday-falls-in-x-part-of the year thing. I also skipped first grade when the teacher discovered I could already read, so I was spared too much time with those goofballs Dick and Jane, and their idiot sister Sally. Since my birthday is in June, I was sixteen when I graduated from high school.