Developmental research is a study focused on the progressive changes that occur as an organism develops. There isn't any way to reverse the changes that happen when you grow up. You can't take an older person and hit the reset button to revert them back to childhood to see how raising them in a different time will change who they are. It'd be nice, but unfortunately we are stuck experiencing time in one direction.
As we just discussed, there are magnitudes of difference between a few years of growth. When looking at individuals along this continuum, a researcher wants to know what changes occur. A scientist is not satisfied knowing 'Things are different.' A researcher wants to know 'What is different?'