Final answer:
Edmund Mortimer was a threat due to his Plantagenet lineage, providing him with a legitimate claim to the English throne, which made him a potential rallying point for factions against the reigning monarch.
Step-by-step explanation:
Edmund Mortimer was considered a threat despite his youth due to the potential claims he could assert based on his lineage. As a descendant of the Plantagenets, which was once a ruling dynasty of England, Mortimer had a legitimate claim to the English throne.
During times of political instability, any individual with a strong hereditary claim posed a threat to the reigning monarch, especially if there were factions dissatisfied with the current rule that might rally around a young claimant. Moreover, the youthful age could be used to gather sympathy and support from those who favored change or were looking for a figurehead to represent their cause.