I'm recovering from having my spine almost completely destroyed in a car accident (it was rebuilt with titanium and I am, thankfully and miraculously, able to walk and take care of myself now), and for a long time I didn't know how to respond to confrontational, usually older people asking me why I was getting in/getting out of my car that I had parked in the handicap zone. I really don't like having to justify my handicapped placard to anyone, but these confrontations were a bit shocking and so for a while I'd go into a long discussion about my health and recovery. Recently, though, I've found it works well to simply say: "Sometimes bad things happen to young people, too."
Does anyone have any other suggestions of things to say? I don't want to appear rude, and when imagining it in my head, "What an interesting assumption" might come off as condescending.