Why do adolescents need 9hours of sleep?

Developmental : For the same reason 2 year olds toilet train, and learn to speak, newborns sleep 18-20 hours, 5 year olds use potty language, etc, etc. It is a developmental imperative. Older folks need only about 4-6 hours of sleep. So as you mature, you need less and less sleep.

Sleep: Since birth to growing up the sleep amount decreases, until adulthood is reached when we need 6 to 8 hours of sleep. Children adolescents need more sleep to help the brain and body grow. For example, when you sleep enough as an adolescent the brain produces growth hormone which helps height muscles etc.