I have 5 boys ages 14, 11, 8, 6, and 2 1/2. They all go to bed at 8pm. They start the bedtime routine at 7pm, showers or baths if their needed, eating, and brushing and in bed by 8pm with the lights out.
Not only do young children need their rest but so do teenagers! I also need a few hours of quiet time in the evenings just to decompress for awhile.
The boys were all bickering/argueing/fighting amongst themselves one night so I made them go to bed with lights out at 7:30 and I could so tell a difference the next day, it was much better!
We also have a rule that if they wake up before the rest of the household and don't want to stay in bed, they may get up and do a quiet activity like reading, puzzles, or silently playing with a toy. My middle boy is in 2nd grade and that is definitly old enough to understand this concept. My 6 year old follows these rules as well and hes only in Kindergarden.