It looks like late wake times peak from ages 14-19. There's a well-documented circadian rhythm shift affecting teens.

Hence schools rejiggering (and California requiring by law) later start times for high school students.

At the school I teach at, middle school students were taken along for the ride, because at our school we need to be on a common schedule (crossover teachers, MS students in HS classes). Of course, on the flip-side, no one wants to push it too late because of things like sports and outings with family wanting to have some daylight after school; we ended up starting 30 minutes later, but also shortening the school day by 15 minutes (and doing some clever schedule changes to claw back most of the instructional time lost).

> Why would you want to change starting time based on age?

Because there primary and secondary education utilizes a single set of busses, and nobody is volunteering to pay for duplicates just so everyone can start as late as you'd like. Locally, this means our elementary kids go first, then the high school kids, then the middle schoolers.

Anecdotally, this is a good arrangement. The middle schoolers really need the late start. The high schoolers are a bit more mature and can handle a slightly earlier start, and generally have an increased amount of extracurricular activities competing for daytime.