The answer is: working-memory space available for constructive use.
Working memory space would provide temporary storage for a certain new information to be stored before being moved to long-term memory.
This mean that older children would forget less information compared to younger children because they have larger storage to keep that information (assuming that the number of information is the same and given at the same rate).
Older children can learn faster and remember more than younger children because older children typically show a significant increase in working-memory space available for constructive use.
