IMHO, the reason older kiters get injured more frequently than younger kiters is that the vast majority of people don't maintain their fitness and health through the years. That's also the reason most pro kiters are younger, by the way. By the time most people are in their 30's and 40's, they've let their bodies go to crap. They start to feel old and begin to believe that age and their aging bodies are a limitation.
For those lucky few who recognize (early on) the importance of a lifetime of consistent fitness, and who maintain a belief system that opens the door to the possibilities that others find impossible, age is really not a limitation. Especially in kiting. I'd agree that age is a limitation in certain other sports (e.g., sprinting). But the nature of kiting allows older athletes (who are fit and believe they can) to compete on an equal footing with younger kiters.
Most people don't have the self-discipline and/or desire to maintain their health and fitness for a lifetime, and even fewer are able to overcome the paradigm of age as a limitation in physical activities. But some do, and they excel not only in kiting but also in many other sports: e.g., Dimitri (doing pretty well at age 48), Tom Brady (NFL, age 40), Satchel Page (joined his first major league baseball team at 42, pitched his last game at age 59), Gordy Howe (National Hockey League all-star twice after turning 42, played in the NHL until age 52, and even joined an IHL team (Detroit Vipers) at age 69). IMHO, it's all about (1) consistent fitness and (2) belief.