[Edi]

It's not a problem with undeath, it's a problem with lifelessness.

The aging mechanic priority order is 1) lifeless, 2) undead, 3) demon, 4) other, so lifeless undead are considered lifeless first, undead second, and aging mechanics are applied accordingly, which is why lifeless undead do not get maxage increase from death magic, but earth magic, and also why they suffer from old age afflictions. Non-lifeless undead should be unaffected. Unfortunately, a good many undead creatures are lifeless, among them all liches and the Ashen Empire Ermorian priests.

The problem would probably go away once and for all if the priority order was changed from the current one to 1) undead, 2) lifeless, 3) demon, 4) other.

EDIT: amended last statement to probable, because I do not know for certain. Earlier unequivocal statement was a mistake on my part.