Re: regular high school graduation age
In some states, if not all, the cut-off for when you're expected to start kindergarten is different than it is in Canada. Here, we use December 31st. So if you turn 5 before December 31st of the school year, you're supposed to enter kindergarten. That's the regular pattern. But in the states (I think most of them) the cut-off is either the start of the school year, or September 1st.
It makes an obvious difference for people born between September and December. In the states they start school a year later than they would in Canada. So, you get guys who are turning 18 early in the year during their senior year. Then if they get held back a year, they turn 19 really early as high school seniors.