It's not impossible, but unless you are a great athlete, incredibly coordinated, and have a lot of time and money (to play and to pay for coaching so your technique is always developing in the right direction), reaching 4.0 or 4.5 is more realistic, and even for that, you have to be a natural. It's the playing matches part that is difficult to master. I've taught people to hit consistent top spin forehands from the service line in the first hour of picking up a racket. So they think, "wow, that was just 1 hour, I bet I could be awesome in a year!" But they don't get it. It's like me going to the park and shooting 8 out of 10 free throws and thinking "I play basketball once every 5 years... I must be a natural because pro players have been playing for years and 90 is a pretty good free throw percentage..." It's not really like that. Hitting the ball under very controlled conditions is relatively easy to master. But actually playing tennis against a skilled opponent is a totally different thing, the skills to move up the NTRP ladder become exponentially harder. Getting from 2.5 to 4.0 is probably easier than getting from 5.0 to 5.5. Or think of boxing, maybe you can shadow box pretty well. But then you get in the ring, you realize technique isn't enough, there's all these things you must know that can only come through experience. How long would it take a 43 year old to become a gold glove level boxer? It may never happen. 5.5 are generally former college or junior players of note. So another way of looking at it is could a 43 year old guy who never played before become a college level (insert sport here) in 5 years? And tennis is way more demanding in terms of skills than practically ever other sport. And movement figures into it, since 5.5 is a performance based designation. If an older guy claims to have 5.5 skills although a fit 4.5 grinder would beat them easily. Nah, ain't buying it, son. Bad knees... long lay off... whatever. No excuses.