The Common Core State Standards (6.SP.A.1) describes it like this:
... one that anticipates variability in the data related to the question and accounts for it in the answers. For example, “How old am I?” is not a statistical question, but “How old are the students in my school?” is a statistical question because one anticipates variability in students’ ages.This statistics standard is slated for 6th grade students1, but I recently heard a more effective explanation of what a statistical question is that will be relevant for middle school, high school, and college statistics. It also defines what makes a statistical question a good one, which the Common Core definition does not.