Do you now operate the machine(s) you are expecting to program ? I started cnc as an operator and read the machine manual during cycles, but these days that might be a luxury.
As far as being 55, it shouldn't matter. When a person stops learning their brain turns to jelly, good on you for wanting to expand your skills.
A link to something hands on and kind of fun
GWizard Editor: A G-Code Editor and Simulator
Machinerys Handbook has a programming section and then there's the Peter Smid books but that's some pretty heavy reading
Anyone who says "It only goes together one way" has no imagination.