I accept the getting older thing has some sort of impact and to some extent can be mitigated...and certainly life/health events = step down or up.
Also in play though, at least round here, is the worsening state of the roads, the road furniture, crossings, lights, no of cars, tractors, lorries. Sometimes gates and sometimes lanes that you need to pull over for cars to pass just for self preservation. All these thing drag average speeds down. Perhaps the thing to notice is the ‘steady state’ speed ie not hill, no interruptions periods of the ride where your travelling speed might be say 18mph and you could notice that is always around that vs getting slower/harder.
The other thing for me is motivation with might change with stages of life where your more focussed on enjoying stuff that pushing boundaries and subconsciously your just not up for it.
i could do with an out front device, let’s call it a YeeHa, that measures those factors that contribute to enjoyment, socialising , experiencing stunning vistas, general health etc etc 😂😂