My kids are 10,12,13. Bedtime during the school year is 9:00.
In the summer, it depends what they are doing and what's going on the next day. I try to get them up around 9 am or nothing gets accomplished in the morning, like chores and school work. At night, if they are just sitting around watching tv, bedtime is 9:30 or 10. There's not much on at that time that they need to be watching. My teenager will stretch it occasionally to 11 if he's on the computer or out with friends.
I know they haven't had enough sleep the next day when they constantly whine, moan or pick on each other. Or if they lay around the house like lumps. That earns a 9 oclock bedtime. I find that everyone gets along much better when there is an actual bedtime.