That's not necessarily true. All we know is that he stopped aging "because he went through the portal" or some excuse. Maybe even if he goes back in time, he still won't age. Maybe in the current time (the future) there exists another Jack, that went back in time, failed to kill Aku in the past, but waited until the past him (our current protagonist) lost the sword so he could retrieve it and finish the job.
But all this time travel stuff is annoying to think about. It's a cartoon, the authors are gonna do what they want to do.