Isn't there a confusion here between how readily your fitness responds to training, and whether or not you can ever be fit?
Relatively, I am already pretty fit irrespective of my age, though one can find fitter people. So I can easily beat our neighbour, a lean 20-something with a super lightweight carbon bike, up our hill. But a few months back I was dropped on the same hill by two middle-aged guys who seemed rather serious about their cycling.
I was really looking for age-related advice, or experience from other perhaps older guys. Should I expect performance to plateau at this age, or is there still some point in ratcheting up training intensity?
I'm not sure anybody can answer that.
Or rather I suspect nobody could answer it with any degree of accuracy, since nobody knows your DNA, or adaptations. Theoretically, you should be losing muscle mass, and some degree of aerobic capacity, as you age. However we all age at different rates. Also, my theory, or daily rant, is that age related decline is only inevitable once you've reached 100% of your potential at a given age.
So for what it's worth, I'd say No, you should not NECESSARILY expect to plateau, unless you really have maxed out your potential.
Also factor in training methods, nutrition and rest. You may have reached your potential given your current strategies, and in order to progress, you may need to mix things up a bit, look at improving nutrition, changing your rides. It is highly likely that if you are only repeating your commute, that you will not see any improvement, whereas if you vary your training, you will force adaptations. Your body will long since have adapted to your standard commute, so you almost certainly have plateaued in that ride.