The yellow eyes are the result of a Force user (Jedi) turning to the darkside of the Force. This is the first visible sign that can be outwardly seen by anyone and not just sensed by another Force user. Later signs include but are not limited to; A corpse like appearance, physical decrepancy, and bodily decay. Weilding the darkside of the Force prematurely ages any Force weilder and thus can rapidly shorten their lifespan. Only those of more advanced knowlege in the Force are able to use dark arts to prolong their lives. Some darkside Force users (Sith) have needed to resort to powered exoskeletons in order to avoid being bed ridden.