While I've done some cycling much of my life, I didn't get serious until I was 56. First century ride when I was 58. Since I only got serious as I got older, my biggest cycling milestones (distance, hill climbs) coincided with my aging milestones. Been very fortunate thus far (knocking on wood) with health that I'm able to still longish distances - although not motivated to do centuries any more.

I don't generally over think about age but have to say, 72 (my current age) does feel like a pretty big number!