There are many medical reasons. Stress is probably the biggest one. But as you get older, the little head no longer has to do all of the thinking like it used to. Other things take it's place. I still open the door of the car for her, the door going in or out of a building, hold hands as we stroll along. After a while, sex isn't all that high on the list.
Like the saying goes, women are mental, men are physical.