Only once was I treated as a mentally handicapped person. It was now 24 years ago but I remember it like it was yesterday. I went to a local milkbar in the town I went to school to buy a drink or whatever with my friends. The person behind the counter was probaby a grade or two lower than me but honestly thought that I was mentally handicapped. Although it hurt like hell, I didn't get angry at her or anything, I just calmly asked how much the drink was and when she realised that I was just a normal (for lack of a better word) person, I could tell she regretted treating me that way and apologised. It was the first and thankfully last time that ever happened. I fully understand what the OP is talking about.