For us it's always depended on what time they need to be up. My son needs about 10 hours of sleep.
If we're getting up at 4am, he goes to bed at 6pm (our winter schedule)
If we're getting up at 10am he goes to bed at midnight. (summer schedule)
If we're in S. Italy (where one naps from apx 2pm-5pm every day) he goes to bed around 1am and gets up about 8am.
Most elementary aged kids need 10-12 hours of sleep.
IN THE US present day: Most get up between 6am & 7am for school. Which means most go to bed *around* 8pm
IN THE US pre WWII, most families ate dinner around 8pm and kids were in bed by 10pm.... because (wait for it) they needed to be up by 8am.