I read an article that stated that at 50 yo you loose muscle mass and VO2 uptake for the rest of your life. I am 72 and definitely see the difference, in y 50's I could ride a 5 hour and change century; no more. Don't even do those any more, too long to recover, but ride 5 days a week and enjoy it tremendously. Don't sweat the small stuff, just enjoy your ride at this age, all relative.