A few interesting studies, tables, etc on age-related performance declines:
https://fairmodel.econ.yale.edu/aging
I haven't given it a thorough read yet, but one table lists predicted declines from age 40 to age 95. I plugged in a couple stats. I trained seriously at age 43, running 17:10 for 5K. If my math is correct, that "predicted" I was capable of 16:40 at age 40, which in turn would predict 20:00 at age 59. I ran 19:13.
I'm not sure if, or how, this table differs from the Age Grade calculations. I think both are based on world-best performances.
Regarding cold weather, I feel much less cold-tolerant in my old age. Well, at least the anticipation of heading out into the cold is a lot worse -- and more challenging to overcome -- than in my younger days. Not so bad after I actually get out there.
This past year was the first I had ever tried a treadmill. There are a few advantages, beyond escaping the cold and ice. It seemed to help my patience and concentration skills. And allowed me to focus on form/cadence. And with an incline, it reduced stress on my knees.
But the best thing is the reduction in laundry. Outside, I donn a long-sleeve shirt, heavy cotton sweats, maybe tights as a first layer, cotton gloves, ear-warmer and hat ... a pile of sweaty clothes after a single run. I probably overdress, but it's the only way I can talk myself out the door initially.
Looking back on 2018, I think I set a personal record for mileage in a calendar year. Still, only an average of 36 mpw. But many weeks at 50 or more. During 45 years of on-again, off-again training, I just never maintained more than a few months of consistent mileage.