HEY! I don't wear rubber pants! And the helmet just makes me look cool.
The mentally challenged are always a touchy subject. Most people feel the need to defend them because they think they themselves are incapable of their own defense. This ordinarily is not the case, and in my experience most have found this more insulting than anything else.
I used to go to school with a kid who was labeled retarded. We took goverment and public speaking together (a lot of arguments were had during class, and I mean a lot). Sure he was a little slow on certain things, but hell so am I, but it affected his speech and a few of his motor skills. So, when anyone first met him, they treated him like Rain Man. This pissed him off to no end, and would often rant at me for extended periods (and often uncomfortably close to my face - bad breath) about this. Now he was majorly into his Christianity, and he was obviously a future republican, so we would often get into loud discussions. Everyone who knew us, knew that even though we were friends that we were both on opposite ends of the political and religious spectrum, so we both usually ended up on opposite sides of discussion groups. As you all can attest, I like to play devils advocate, often taking the other side or less popular argument just to test it and yours. It is something of great interest to me. Anyway, now that I've got that out of the way...
One day we were both ended up at an event together - neither of us knowing the other would be there - and ended up at a table with some other random people. Like always someone comments of something they read, or in the news and I somehow get involved in the discussion. Now I knew that the second I finished my statement that Isaac would come and jump into the conversation, because he can't stand to let me off the hook for anything. He does just that, but because of his speech impediment, and the arm and shoulder ticks, it takes him a while. Now I had heard him make an argument similar to this before and knew where he was headed with it, so I cut him off and started to josh him about it. The people at the table didn't know that we knew each other, or that Isaac may have had trouble doing certain things, but was honestly much smarter than all of them. They all looked like I had just crapped in the hot tub, or punched an old lady holding a baby. So, Isaac, being the douche he is turns to them and says, "I know he's retarded, but it's rude to stare".
tl;dr? Most people are idiots. Ignore them whenever possible.