A child's bedtime is of concern to the entire family because tired, cranky children have the potential to upset the entire family's well-being. Parents also need downtime away from children to reestablish their own intimate bonds. This is not "getting rid of the child," but putting the child down for much needed rest while parents experience one-on-one relationship interaction that keep their marriages healthy which also benefits the child(ren).
There should be a natural evolution of a child's ability to govern their own sleep. As you mentioned, children under the age of 9 or 10 do not generally have the ability to maturely evaluate their own sleep needs and make good decisions. With puberty comes an extra shot of seratonin that makes teenagers naturally want to stay up later and sleep in later until the morning. It's a biological reality that our society does not acknowledge because Jr. Highs and High Schools often start even earlier than elementary schools do. Because many children lack the maturity to be able to fight their own biology on sleep, it is necessary for parents to intervene sometimes.
Our rules for our boys during the school year:
Six year old's bedtime routine starts at 8, lights off at 8:30PM. He gets up at 5:45 so he has extra time to get ready for school and the bus comes at 6:30.
15 and 16 year olds have to be in their rooms after 10PM during the school week unless there is a good reason to change the rule (example, they don't get home from an away game until 9:30). They don't have to sleep, but there aren't any tvs, computers or phones in their bedrooms at night. So while homework or reading is optional, they generally don't partake for more than hour which makes it far easier for them to be up at 6:30am and out the door at 7:15am. If they choose to read until 2am, I'm not going to interfere with that. But the lack of things to do make them less likely to go that route.
On weekends, the older two have to be home by midnight and quiet. There is no established time they have to be in their rooms, but if they're loud and irritating they and whatever friends they might have over will get sent to bed. The rest of us still need sleep.
During summers and on over breaks, the six year old is in bed by 10pm. The older boys have to stay quiet after 10pm. They don't have to be in bed, but they can't be in my or my husband's hair (we generally go to our bedroom around 10 too because we have to work the next morning) or causing a ruckus that might wake the six year old up.