I'm sure it differs from person to person, but I'm just trying to get a handle on what's reasonable
If you understand that people are different - then you know they "age" differently.
My experience suggests that declining athletic performance is a result of whatever "body part" loses it abilities. That can be the heart, lungs, knees or back - or blends of many more ailments.
Some medical article once discussed the concept "age scoring" of various human organs. Different body tissues age at different rates. (brain scores, calcium scores bone scores etc..)
In my case "the brain" went first - followed by my back and heart. You can keep on riding until some part of your body tells you to stop
-not the calendar.