What's the best sleeping position for a 64 year old male who suffers from chronic, but mild lower-back disc pain?

As a 47 year old. ...With chronic back pain, I have found it very helpful to sleep on my side with one pillow between my knees. I find it helps maintain by spines natural curvature. You can also try a "body pillow), which is a longer pillow designed to help people who like to sleep on their sides.
Back pain. There is no right or wrong answer -- find a position of comfort and try to stick with that -- an adjustable mattress may be of some help.