This manga makes me wish that we don't look for ways to extend our lives, but to strive for living all of our days with healthy bodies and at the peak of their youth. Fuck immortality. Quality of life would be better, studies say the human brain can't really survive without problems past the 120 years.
Imagine if your new son was younger than your grandson...
Also, because their lifespans are restored, they, in theory, could have a kid when their great, great grandchildren are born, meaning that they'll be so removed that it might not be considered incest, which is a weird thought.
When they get old again, this repeat except they become high-schoolers. then again and they become middle schoolers. Then again, and they are back to elementary school xD then not again cause too young LOL.