[Mithalwen]

I agree with Inzil that young is relative and means more when they were still vigourous and numerous rather than the nearly spent force of the War of the Ring. Also as Morth points out Denethor was a scholar, a master of lore. Even though peoples fade and kingdoms fail their artefacts often survive one way or another. Some may well have made their way to Gondor and ended up in the treasure houses of Minas Tirith which of course the steward would have access to. Valuable stuff manages to travel even in ancient times. Also since Gondor and Arnor had shared origins their style of craft were likely to show their shared origins even if their paths diverged.