Many of my older students ask me, "Do you think I'm too old to learn to play tennis?" My answer is, "No, you are never too old to learn!" No matter what your age, or how long you have been playing, the sky is the limit.
Tennis, like many other sports, is learned by repetition. As long as you are repeating the correct skill or form you will eventually be able to retrieve this information in a competitive format.
From my personal experience, I really enjoy taking a beginner/intermediate or a group of beginner/intermediates and instructing them. They tend to be eager to learn and are very enthusiastic. Also, they put in the practice time and thus enjoy positive reinforcement. As a player advances in skill, some tend to rest on their laurels. They tend to play more matches, but seldom work on particular weaknesses or adding more and new weapons into their game. It is quite common that a newer player to the game can overpass a more experienced player in a short period. Even the top Pros set aside time from their touring to rest and take fundamental lessons from their pros.
It is easier to learn "good skills and habits" than to break "bad ones." In almost 30 years of teaching, I have never found anyone that I could not help to improve his or her game, attitude, or footwork. This is the reason why millions of people find tennis so much fun and challenging at the same time. Do not let anyone tell you that you are too old to learn to play tennis. If your lessons are seemingly repetitious or boring, it might be time to find a new instructor. Look for someone who motivates and pushes you to fulfill your potential.
Good luck on the courts!