All perfect praise be to Allah, The Lord of the Worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Washing the face in ablution is obligatory as per the Quran, Sunnah, and scholarly consensus. The boundary of the face is, vertically, from the hairline to the bottom of the chin, and, horizontally, from the ear to the ear. Therefore, it is obligatory to wash the face from the forelock at the top of the forehead of a normal-haired person.
However, scholars underlined that the face should be washed beyond the mentioned boundary during ablution in order to be on the safe side and make sure that the whole face is properly washed. This is with regard to the upper limit of the face. As for the lower limit, which is the chin, it is the end of the jawbone where the lower teeth grow (the mandible). Whatever is beyond it is not considered part of the face and, accordingly, it is not obligatory to wash it during ablution.
As for the hair next to the ears on the sides of the face (the sideburns), it is considered part of the face and therefore it is obligatory to wash it during ablution. The hair against the top of the ears, on the other hand, is part of the head and should be wiped over along with the head.
Allah knows best.