I haven't seen a serious study, but I've taught Scratch programming to kids in the 6-10 age range and found that they were all able to grasp loops if presented in an appropriate way. Similarly, we didn't talk about functions exactly, but each Sprite in a Scratch program can have multiple blocks of functionality triggered by different things.. i.e. functions. By the end of a year-long course, all of the students understood at least the basic idea and several of them had a pretty deep understanding.