I seem to recall (and this may not be accurate, it's been a few years since I've read the books) that Aragorn is 78 when the Lord of the Rings takes place. Not all humans in the Tolkien's world as a long lived as he is. The Numenorians were very close with the first Elves to come Middle Earth and learned many things from them. The relationship with the Elves eventually changed them, they became stronger and longer lived than most humans. Not many pure blood Numenorians are left, but the ones that are still around, like Aragorn, can live for a couple of hundred years or so. After the Lord of the Rings, when Aragorn becomes king, he lives for at least another 50 years.