The Dunedain were descendants of Elrond's brother Elros, who chose to be counted amond Men, rather than Elves since he and Elrond are half man/half elf. Since Elros was half Elf, he lived three times longer than normal men, and died at the age of 500.
Since the Dunedain were descendants of he, they had the same long life. But after comming to Middle-Earth and mating with the humans already there, their long life diminished. However some still retained the gift of long life. Aragorn was 87 at the time of Lord of the Rings, and died at the age of 210.