I started riding on the street at 15. Family genealogy says I would live to 78 naturally (older brother, father, both grandfathers made it to 78). My goal was to ride until I was 70. Some things start declining at 40, more at 50. more goes wrong at 60 and even more at 70. I made my goal of riding till I was 70, but was having some more serious issues (heart, prostate etc) to go along with the general muscle aches/weakness, eyesight, hearing, memory, reflexes slowing down etc. Doctors and drugs have kept me going and riding. In 3 months I turn 74 and I'm still riding 300 days+ and 20,000 miles+ a year.
Some people are blessed, few ailments, brain sharp as a tack, eyes like a hawk, can work hard all day into their 80's, but the reality is for most of us it's not like that ..and it's natural. Our bodies are designed to eventually fail.
I rode to a Doctors appt the other day, and they were asking me general old people questions they ask these days, do you fall a lot, are you depressed etc, and I told them no I was good and wondered why they were asking.. and they said "we ask everybody your age.. some 73 year olds still ride motorcycles (while pointing to my jacket and helmet), while other 73 year olds are riding walkers".
Enjoy every day you have, ride as much as you can if that's what you like, and realize that it's natural to slowly lose abilities as you age.