To me it makes a lot of sense that he might have had an injury either in the army or doing something active, like a fall off a horse or similar. There's no proof, of course, but it's also not terribly likely that a 35 year old actually has an infirmity like this unrelated to an injury. As others have said, I've seen this myself. My husband broke his ankle (going off a jump while skiing) at age 28, and it's bothered him during changes of weather ever since then. At age 35, he would certainly have complained about his ankle aching, and a teenager listening to him might have thought 'old man' when actually it was because he was overly active that it happened in the first place!

And I think that's the important point to gather from this scene--not that Colonel Brandon did or didn't injure himself in the army or that he was or wasn't a sportsman, but that Marianne, in her narrow-mindedness, cannot imagine that there could be any other possibility other than what she supposes to be true.