A jury of your peers

So like, if you are in the US and you are on trial, and you are retarded, do you get a jury of retards?

No, peers means people living under the same set of social values as you. If you're retarded non retarded people within your cultural region are still your peers. Of course, if you're retarded you don't stand trial in the US because part of US' social values is empathy and fair justice.