Aragorn, son of Arathorn, is unique among the men of Middle-earth, particularly during the events of The Lord of the Rings. The Ranger's bloodline, which includes some elvish DNA, is well-known for creating long-lived kings. See, way back in the beginning of the Second Age, the patriarch of his family tree — Elrond's brother, Elros — chose to become a mortal man. (Compare that to Elrond's choice to become an elf. Yeah, it's complicated.) Even as a mortal, though, Elros lived to be an impressive 500 years old.
Over time, his descendants had shorter and shorter lives, but they still add up to multiple lifespans compared to other men. So, how old does that make Aragorn, then? Well, he's 88 years old when the Ring is kaput — he tells Éowyn that he's 87 in The Two Towers film, but his birthday is the same month that the Ring is destroyed, pushing him up to the 88-year-old mark in the process.
This puts the ranger head and shoulders over the younger members of the company. He's almost twice as old as Frodo, and he's triple Pippin's age. While he remains a hearty warrior, it's Aragorn's age that makes him a particularly good candidate as a leader of men and the eventual high king of Gondor and Arnor. He's experienced (seriously, the guy fought with both Denethor and Théoden's dads) and knows how to be patient. Once he becomes king, Aragorn goes on to rule for 120 more years, dying at the ripe old age of 210.
