halacha - Does Down Syndrome make one a Shoteh?

I had a very strange experience and I would like to know what halacha thinks.

I was in shul on Shabbos, and an adult with Down Syndrome ripped by opening the box of tissues. I was not going to say anything, but I guess he noticed my disapproval of what he had done. Now I remind you I said nothing. He looks at me with a straight face and says, "Don't worry; I am a Shoteh." Now, I want you realize that he was intelligent enough to read my face and understand what I thought he did was wrong, yet I still want to know: Is he actually correct?