An immature prefrontal network seems to be responsible for the trouble that children (here age 8-12) have with manipulating information that they're keeping "in mind". 8-12 years have an easier time "spitting back" information (rote memory) - it's just they break down if they have to manipulate what they're keeping in mind.
Every parent and teacher is accustomed to this - it when a student can repeat back what they've seen or heard, but can't analyze it or apply it to some other purpose.
In the figure below, look at how more efficiently even the worst 18-25 performers on the working memory task were able to activate their dorsolateral prefrontal cortex on the backwards digit task, compared to the 8-12 year olds.
The practical implications: when teaching 8-12 year olds and having them perform task that require manipulation of information (applied problems, multi-stepped math, etc.), it is very important that working memory demands not be exceeded. In order to work with data, most preteens can't keep that much in their heads - they need more written explanations and the information should be "chunked".
Children Struggle with Working Memory Manipulation