The Einherjar are those children chosen to pilot the Aesir into the battle against the Jotun. Those chosen by the gods to fight on the last line of defense between humanity’s survival and its extinction.
The one thing in common between all the children chosen is that they are born after 2015. The reason for this is a particular brainwave found in the brain function of all people who were alive before that date.
The ability to pilot the Aesir is not universal to those children born after 2015, and is in fact somewhat rare. While some are raised from birth by the NERV organization directly to pilot the Aesir, they generally come from a varied background and may have originally been citizens from the cities within the domes, or rarely from the vast plantations surrounding them.