Every one under 12 should be in bed by 9. Young people need their sleep, and when they are deprived of it, they set themselves up for failure in the future. They do not pay very much attention in school, and they are cranky and irritable with their peers and their parents.
Children need to get their rest, so they should go to bed at a reasonable hour. If they stay up too late, they wont be able to focus on their school work the next day. Also, there are TV programs on their air after 9 o'clock which are inappropriate for young children.