User blog comment:Xilinoc/Ch. 541 - THE BLADE AND ME 2/@comment-7924893-20130612200535/@comment-5112778-20130614014015

Don't forget Isshin stated that Yhwach absorbed many half-blooded Quincy to get back his power, That maybe the reason that he's still alive after 1000years and looks like he only aged a few.

Tho these are all theories.