And South Carolina goes one better,

[It Aint Necessarily So]

K12 is ages 17-18?

Just the 12 part, which refers to 12th grade, not the K part, which refers to kindergarten. It should be written K-12 to express a range.

In my day, elementary school was K-6, and the average age of the kids was from 5 yo (K) to 11yo (6th grade), junior high was 7th through ninth grades (ages 12-14), and high school was 10th through 12th grades (15-17), although 18 was a common age to graduate high school.

Joke: They named the neighboring junior high and high school after country music legend Hank Williams and his son Hank Williams, Jr. They were both Hank William Junior High.

 

[Pogo]

What makes this so interesting is the population over the same period time:

View attachment 93653

Yes, it appears to be an exponential curve that has or is reaching an inflection point. A typical growth curve with a secondary input.

My experience says about 1960.