Your marriage sounds a like mine (ours), with the exception that I had girfriends and went to prom. Twice.

Physically, as far as keeping it up as the years pile up, #1 is going to be nutrition. #2 is exercise. #3 is probably genetics. If you do sports, limit alcohol and the sausages you eat, your body will be in much better condition to fill your own sausage at the drop of a hat (or panties).

Mental and emotional fitness are arguably just as important. As with your marriage, my wife and I love and appreciate each other very much. It is easy to get in the mood if your relationship is filled with love and mutual respect.

If your sex life is based only on physical attraction, then the older you get the more gravity is going to fuck you more often than you are going to fuck each other.