Fly,
I've hunted elk for a lot of years and I'm about the same age as you. I really wouldn't worry about the rifle and load at this time. A guy I know shoots them with a .50 PRB and seems to do well with it. I would spend time selecting an area and more time getting into shape to hunt. The Rockies are man-killers if you aren't ready for them. Each of the last two years, we have had guys come with our group who couldn't walk from the camp up the first ridge. It is sad to see guys who have put a lot of time and $ into getting to hunt and then can't handle it physically. For your first hunt, you might want to look into a guided hunt or drop camp hunt. It will give you a taste of what you need to be ready for. At a minimum, I definately would arrange a local, with horses, to pack the elk out - BEFORE YOU HUNT.
For what it is worth, I chose a .58 flintlock with prb and 120gr of GOEX FF.
Best of luck with whatever you choose and enjoy the process. Elk hunting is a wonderful sport!