Age is just a number.
If you want to do it, I mean WANT to do it, you will.
If you would like to do it, then you won't.
Performance at any age is largely a mental thing.
That sounds like a series of those quotes on posters that are put on the walls of gyms or call centres.
Age is a hell of a lot more than just a number. People have peak fitness and peak performance levels which they build up to and gradually drop down from. The rates of increase and decrease vary between individuals but they do exist.
I agree that performance, at some level, is a mental thing in that it takes some will power to maximise it and also to minimise the rate of degradation, but it is also a physical thing and performance, absolute and relative to past history, will eventually degrade with age.
It gets to me sometimes that, at the age of 75, I know that I am getting gradually but surely slower, and take longer to recover from a heavy workload, but the alternative is far worse. The only difference between the way I do things now and what I was doing ten years ago is the time that has passed and the fact my body has aged.
I hope I am still riding in five years time, even if it means an e-bike, but I guarantee I will be slower without an e-bike.
One thing I would add is "use it or lose it". Once you stop a sport or form of exercise it gets a lot harder to take it up again. I played squash for forty years until I was 60 and was OK at it, but when I was 68 some old opponents over the club persuaded me to play again. Big mistake, it had all gone.
i dont know what the future holds but am hoping for steady improvement with strength and endurance at least for a few years. If i can only reach the heady heights of 40-50 miles over varied terrain thats fine by me! 
