What can be the causes of the difference in student performance?
There is a point, somewhere around where puberty starts but I don't think identical, in which the brain changes its structure. It is much more capable of abstraction after that point than before. Young kids don't do a lot of generalization, for example, adults do. So, perhaps the study just captures part of that. 

I don't have it available, but I think there is a lot of brain science around that structural change. A simple search may turn up some of it. We aren't born fully formed either in mind or body. 

An implication of that is that how and what you teach toddlers and youngsters isn't the same as what and how you can teach teenagers. Also, every person hits that change at a slightly different age and it isn't instantaneous any more than any other brain change is. For the youngest, make it visual and tactile, even aural.