In randonneuring it seems like the plenty guys keep on through their early 70s, but hardly any are still putting up big miles after 80. Of course everyone hopes to be that outlier.
At 63 I'm still riding grand brevets and feeling plenty strong, but some things are noticeably changing. Nighttime vision, low speed balance, heat tolerance. But then rando isn't supposed to be easy.
To op, just ride today, don't overthink it.