Progesterone, a.k.a. "P4", pregn-4-ene-3,20-dione, C21H30O2), levels decrease with aging.
Women in their 20's have high or normal levels.
While women in their mid-30's often have a "deficiency".
Some researchers claim that this was not true prior to the 20th. century,
blaiming such decreases on modern diets and enviromental conditions.