Can't see how a shorter stem would affect the hamstrings, but a different saddle position probably would.

Formulas never seemed to work for me. I stumbled onto my best fit proportions quite by accident with my old 56cm Basso. The ergonomics were so good I copied all the dimensions to my other bikes, even the 54cm frames. Course, I failed to write it all down. Good thing I kept a bike or two!

Regarding physical changes, there are a couple of threads discussing the need for smaller frames as our bodies change. I think most of us need less saddle to bar drop as we get older. Enjoy your age while you can. I just turned 60, and it's beginning to show.