(Looking for developmental dissociations between processes can be a profitable research strategy, but such dissociations may be affected by external events like the transition to formal schooling.)
As a developmental psychologist, I'm primarily interested in answering "how" questions: How do children figure out how objects work, learn the meanings of words, or recognize the beliefs or goals of others? Yet along the way, I can't help interacting with the (less interesting) more descriptive set of "when" questions: When do children show evidence of object permanence, learn their first word, or pass false belief tasks? And in studying any individual phenomenon, answers to "how" questions can be informed by estimates of when a particular behavior is first observed.
But here's an issue that has been bothering me for a while. Our "when" estimates – derived as they are from the behavior of middle-class kids in the US and Europe – are not independent from one another. They are instead highly correlated, because of external milestones in the lives of the children we are studying. Transitions to preschool or to kindergarten are major drivers of new behaviors. Worse, because teachers are active readers of developmental psychology, new school experiences likely involve explicit practice of exactly the kinds of skills we're interested in studying.
One possible example of this issue comes from a lovely talk I heard by Yuko Munakata at the Cognitive Development Society meetintg. Munakata has a deep body of recent work on the development of children's executive function (roughly, the ability to shift flexibly between different sets of behaviors according to context or task; review here). She documents transitions in children's executive function, including the transition from reacting to a stimulus to proactive preparation – choosing the proper behavior for a particular situation ahead of time. To be clear, nothing in Munakata's work depends on the precise timing of these transitions. Yet suspiciously, many of the transitions she studies happen in the same age range (4 - 6 years) when children are transitioning to school, an environment where their executive functions are being challenged and perhaps even trained.
A second example (very far from my area of expertise) comes from a comment made by Kate McLean in a recent brownbag talk she gave at Stanford. McLean studies identity development in adolescents, and she noted a big uptick in the quality of narratives in later high school. When she probed more deeply, however, she uncovered an external driver: late high schoolers were all engaged in the same social ritual: college application essays.
The research in these examples is not necessarily compromised by the presence of external events. But nevertheless, these kinds of events are big factors that might affect study outcomes in ways we wouldn't otherwise predict. From my perspective, I wonder how much the cognitive constructs I am interested in – pragmatics, language learning, theory of mind reasoning – are affected by individual children's transition to preschool, since the period around age 3 - 4 is a time of tremendous development for all of these abilities.
Studies that dissociate age and school shouldn't be too complicated to do, for either executive control or for other constructs. And these sorts of studies might give us some insights into the ways that (pre-) school experiences support the development and refinement of cognition. I recently heard the term "academic redshirting": holding children back from starting school so that they are older and do better than their peers when they finally start. This is a fairly intense (and controversial) strategy for getting kids ahead, but it might create an interesting natural opportunity for studying cognitive development...