From: Becky B.

One of my friends is an RT who specializes in mammography.  She says that between the ages of 35 and 40 the milk producing glands in the breast begin to turn
to fatty tissue.  It is a very slow process and ends when women are through
menopause.  She has shown me mammograms of the various changes through time, but
when she showed them to me I was a student and not in the baby business yet.
I'll see if I can look her up (she moved) have her explain it to me again
with the picture.

Becky B.