And here’s one reason why, from the NY Times:
That’s why [Roberta] Brinton [a noted researcher] says that the timing hypothesis — the proposition that estrogen could bring great benefit to a woman who starts it in her 50s while having the reverse effect on a woman 10 years older — makes sense even though it is still experimental. She and other scientists know there are ways estrogen improves and protects the brain when it is added to healthy tissue. It makes new cells grow. It increases what’s called “plasticity,” the brain’s ability to change and respond to stimulation. It builds up the density and number of dendritic spines, the barbs that stick out along the long tails of brain cells, like thorns on a blackberry stem, and hook up with other neurons to transmit information back and forth. (The thinning of those spines is a classic sign of Alzheimer’s.)
One reason concerns, then, the health of the aging female brain. There’s a second group of researchers who are looking into a similar hypothesis about the heart. With each, the important point is the timing; the effects of taking estrogen in perimenopause appear to be very different from taking them afterwards, to put it roughly.
The article does not have easy answers to the question of what to do. The author remarks, “The personal calculus of risk is an exhausting exercise in the modern era…”