I am 1.5" shorter at 50 than I was at 17. I know some shrinkage is normal, but is the amount normal in a male who was 5' 11.5"?
Not typical. You are correct that we do lose height as we age but this seems extreme. It would be worth an evaluation to see if problems such as osteoporosis or scoliosis are present which both can contribute to a loss in height.
Normal vs abnormal?! Normal loss of height is due to shrinkage of the vertebral discs due to loss of water content and doesn't cause pain. Abnormal shrinkage may be due to osteoporosis or other bone diseases and may cause vertebral body deformities. These deformities can be painful or painless. If you have a family history of osteoporosis you should be checked for this.