Are we over the hill after 40? Recent research says we're not. Rather, middle age has been crafted by natural selection to fill a powerful role in human society.
"...middle-aged people may be seen as an essential human innovation, an elite caste of skilled, experienced 'super-providers' on which the rest of us depend."
Changes in body shape and functioning can be reasonably explained with reference to the new roles middle-aged people play as nurturers of their grown-up children and grandchildren, teachers of skills and knowledge, and coordinators of social enterprises.
Middle age: A triumph of human evolution New Scientist