The ultimate user interface tester is a child, the younger, and least experienced, the better. Since your audience could be anyone, you really want to test with the last experienced group first and then work back towards the rest, and an inexperienced child provides you with not only someone very inexperienced in your app's content and functionality, but also inexperienced with life. They are the ultimate blank canvas to observe how the untaught human mind will naturally interact with your interface elements. Once you see and understand this baseline you can then begin to work back toward the rest of the human race that has been constantly taught and retaught different societal norms throughout our lives.
Having said that, I must commend Google and the Android team for their work on the folder user interface element. Not only does my two year old understand the concept of the folders in that he can tap it to see more apps, but he also understands that it is a storage space where he can store more of his games. He regularly finds other games I've installed on my phone and drags them to his folder on my home screen. Of course this leads me to question my sanity often. "Where in the world did that app go? I must be going crazy!"