I wish to weigh in on the benefits and wise remarks to go to a profesional fitter. Someone looking at you can see what you can't see yourself. I'm over 50 and my fitting ended up being the measurements for a custom bicycle.
Some things I don't have now: I used to have neck and upper back pain after a long ride, don't now, and I ride longer rides. My knees are problem areas and I had to be careful to stay centered or one knee or the other would ache after a long ride. Now I can put very long rides in without knee pain.
From what little I understand about riding physiology you should not weaken as you age if you maintain your physical effort level. With that in mind, other factors that may not be obvious or intuitive could be at play here. For these reasons a good bike fitter can see your position and get you set up correctly considering your core strength, your flexibility and any joint conditions you may have.