I think it is more of an evolved response to predation. You need people awake to watch for threats. Teenagers could do that while adults tend to young kids etc... young kids and grandparents tend to awaken early etc... adults too wake earlier as they age so you reduce the threat window.
Maybe, but I wouldn't have put my teenage self on nightwatch. It wouldn't have been pretty. I'm also getting to the point where I don't think I would trust my father to be on early morning lookout.
Seems like there are enough adults who are night owls and morning larks to cover an adult-only watchfulness.
