Slc2 has given you lots of background. If you need motivation consider this: my mother rode her first Prx St. Georges test this past fall at the age of 71, on a horse she trained herself, . I have a "mature" client, who asked me as she hit 60 if I though she and her mare could someday manage First Level tests. Not only did they manage First Level tests, they went all the way to Grand Prix!
Dressage is fun regardless of your age. It's very much a cerebral exercise, although, as slc2 says, it can be very hard physically, if that's what works for you. The intellectual challenge appeals to many people, and often with age comes the ability to step back and assess situations, and then find other approaches to problem-solving. That often works well in dressage, and it's definitely one of those "slow and steady wins" kinds of sports. It can be a ton of fun, and competition really can be just against yourself and your own improving standard. It's been my main game for 40 years, I got hooked and stayed hooked, in spite of a background riding in every western discipline, H/J, and equitation.