>>100062133Jack couldn't age because time self corrects in the universe he lives in. Aku force him into another time, but he should be dead in that time... only way he could live that long was if he was ageless.
Aku's magic was strong, so he was trapped. So time made him ageless.
Likewise, Ashi couldn't exist in the past. Not because Aku was gone, but because her presence was an impossibility before her birth. So time tore her back to the future.
There, she continues to do battle with Aku. He didn't die in the future. After all, even after they portal-ed back he was able to voice his concern (he should have simply ceased). So he's in a separate timeline. One that the unique portal (the guardian portal) can reach.
>Jack reunites with Ashi, they overthrow Aku, save the future, begin to replant seeds of hope in the ashes of the world. Jack is weathered, but ageless again. Ashi is of Aku, and lives forever as well. Both become demigods of sorts, guiding the Earth to peace and living together in love for all time. 
