Century Babies

Century Babies are humans that were born "with the century," on the stroke of midnight on New Year's Eve of a new century (i.e. 1800, 1900, 2000). As they grew, each displayed certain superhuman abilities and skills, as well as functional immortality. While Century Babies are known to be immortal, they are not invincible, and can be, and often are killed, invariably as a result of the extraordinary lives they lead.

It is theorized that the Century Babies act as an immune system for the planet, their very existence protecting Earth. Each of them supposedly represents an aspect of the century into which they were born. - edited from Wikipedia