How much of a factor is age in improving your game? If you are middle age and doing the same amount of practicing/drills/technique change as someone in their 30s, does your age limit how much better you can get no matter the effort? Will the younger guy have a higher ceiling and more rapid development just based on age? Please factor out movement, quickness and fitness, I'm asking purely from a technique viewpoint.
A lot depends on athleticism and visual acuity and depth perception. As these decrease with age, if you play younger players (of similar caliber), you may lose as your edge in these areas is less.
You can't learn well if you have poor body mechanics/body sense.
You can't learn well if you cannot see the ball well to move. Yes, vision and then footwork is the most important.
Lastly, you can't learn well if you cannot remember. 10,000 (good) reps to hone in on intuitive movements.
I can tell you that I forget more about tennis mechanics and how to hit well as I play more doubles matches vs. hitting with a ball machine. I can hone in on what I need to do with a ball machine. I cannot in the middle of matches. Therefore, I think "hitting" and practicing are key before playing a lot of matches. Many times after a good hitting/ball machine session, I take that into matches and know what I need to do for shots I have trouble with.