It does seem likely that the Jack who is destined to use the portal is older still than the Jack we are currently following, as that Jack has grey in his hair and beard, and his face is lined. Current hypothesis regarding that is this: Jack defeats Aku in the future and then works to restore the world as its king (see: golden crown upon his brow in portal vision), then returns to the portal when he is ready in order to save the past.
A viable theory, that Jack has years to live up to his prophesied image, but:
Pardon the vulgarity, but if Jack has successfully un-fucked the future
while being there, why would he go back to the past that is still fucked? The evil that is Aku has been undone, that was the key part of the original pitch -- going back to the past wasn't a goal in its own right, it was a way to achieve the goal.
But there are those who perished in Aku's conquest and under his rule, I suppose - to save those, maybe it's a reason enough to go back in centuries and fight Aku all over again.