It actually depends on who is saying it. The most common use of "You're too young" is actually well-meaning elderly people, often with a mobility aid themselves, for whom English seems not a first language (these are my frequent seating partners on the bus). I'm afraid I would confuse the kindly old Chinese or Russian person with the "smart" remark, so I just look saintly and say "It can happen at any age"; they tut-tut and say "That's too bad, you poor thing", and that's all right.

From people nearer my own age, I am more likely to say "Oh, you flatter me, but you're too clever for that; I *know* you you understand that disability can happen at any age."

There's also the caveat of what preceeds the conversation. If a casual acquaintance politely asks me why I use the wheelchair, I am likely to say "arthritis"; fibromyalgia is still hanging out under the arthritis umbrella, and it's a quick, simple answer that most people understand. However, arthritis *is* perceived as a disease of old age, so that answer is more likely to lead to a "you're so young for that" sort of comment. I reply that arthritis can strike at any age, smile, and change the subject.