My wife and I have a couple of friends, and our youngest and their youngest get along really well and play together a lot. But so often they invite us over, or we invite them over, and they're early 20's son with down syndrome is there. He's so hard to understand, he makes me uncomfortable, he'll ask the same question 5 times in 10 minutes, and I'd rather be anywhere than around him. 

I spent my childhood volunteering with my dad at a centre for the mentally challenged, so you would think I would be better at dealing with it, but I'm not. Last time he came over uninvited, I basically begged off that I wasn't feeling well, and went to bed. But I had to lock the door, because he tried to come in. Sigh.