D.P.
Just treat it as an opportunity to talk abut the similarities between people!
Kids are really, really accepting.
The first time I encountered this, my son was about 3 and he was "catching candy with other kids at a parade. He didn't get why this O. "grown up" guy was right there with all the kids--he had Down Syndrome. I explained to him in very simple terms that although his body had grown up (so to speak) his mind would always remain child-like due to a condition the man has had since birth. Then I explained that O. class-mate's sibling (that he knew) had Downs, etc.
Just be matter of fact. Don't impart pity--just understanding!