Proving without using ANY cultural references. That way we'll know it's objective and not a matter of past mores and social conditioning. Good luck. Men can stay sexually virile and potent well past 50. Women undergo a complex chemical change as their reproductive system shuts down, in their late 40s. Men do not gain full musculature until their late 30s/early 40s. Peak fast twitch/neural synaptic response, of course, occurs earlier, but brute strength peaks around 40 or so. Women hit prime fecundity around 18-22. Absent any cultural references, pure biology, men maintain their "maleness" longer than women maintain their "femaleness." I think the ever present biological drive to reproduce, couple with the above, account for much of the references of so many cultures.