The 20-Somethings Win! They've got more parietal power(representations, imagery)- and so are quicker and more accurate.
It's great to see functional imaging studies documenting the normal developmental evolution of basic cognitive skills essential to learning. The next step, of course, will be to make sure that we are using developmentally-appropriate approaches and having developmentally-appropriate expectations for teaching these kids.
Developmental Changes in Working Memory - fMRI