Hello, just an addition to this thread rather than a reply - forgive me if that's not the way to join in
51 years old. Tandem and lightish Specialised road bike. Now living and biking in the middle east, originally from UK. As a teenager I was a keen tourist, as well as bringing up the rear in English Schools Cycling Association national finals in road racing, cyclo cross and time trials - but they were kind, they let anyone play who had a bike!
About biking in middle age and beyond - I'm the young new guy in my cycling club here, and I can't hang with them for more than a couple of pace line pulls! 'Just do it' seems to be the truth.
And to share an inspiring memory about fitness despite the years - in 1990 when I was about 36 I trained a bit, and finished the NYC marathon better than I was hoping for, and just before the finish line I saw some press attention. It was for the fellow in front of me, winning the category "males, 80 and over".
Never, never, never stop, as a rock band states!