[Publius]

Aragorn, like the other "lost" kings of Gondor, could live to a very late age and would age slowly. He also had the ability and right to decide the time and place of his own demise. Having descended from the line of Elros of Numenor, brother of Elron, there was the promise of long life and wisdom.

[Romestamo]

In the Unfinished Tales, Tolkien surmises that, at first, the Numenoreans did more than just age slowly. It seems that they remained hale until within the last five years of their life, and died without struggling against their age. Only when the kings started to become greedy for life and power did they become feeble for longer periods at the ends of their lives.