The obvious difference is that in this one, one elderly person (who was always single) regains their youth, setting up harem hijinks.
In the other one, the elderly couple regains their youth simultaneously, setting up heartwarming moments.
Checked 2 final chapters, nahh man it's not gonna work. Harem from underage girls where're MC is over 80 (mentally). I can understand 20+ somewhat but it's already kind of a stretch. + Adoptive children as harem members? I know some works that use this plot, they are usually received with a lot of criticism but they usually try to make a compelling reason to allow most people to agree. (forgot the name but the guy was 40+ at the time)
Old lady declined his advances on the basis that he thought of her as mother while not realising it himself? That's a stretch that will crack any moment, especially if someone older than 20 is reading this. You can fool a teenager who didn't live that long to believe this, because they don't know how people at age 20-25-30+ think or feel, but older people get it.
The most primitive answer: You love your mother, you can adore her (mom-con) but you will never feel sexually attracted to her, you will never want to embrace her and touch her, smell her fragrance and kiss her. (If you do, it's not love for a mother it's a love for a woman).
Honestly though, you don't even need to be 20+ to understand this, i gave this benchmark for teenagers that live in a bubble or have mental disability. I clearly understood the difference in age of 4-5, i liked some females differently and as i aged it just became ever more crystal clear and pronounced. (Maybe seeing porn on TV helped, the things guys did with the women was the things i wanted to try with other women excluding my family members, simply felt some kind of aversion while thinking about it.
So yeah author clearly didn't think through and just cornered themselves from the start to the bitter end. You can be physically attracted to younger girls but at the same time even in my younger age, i had a preferable age of women and it didn't change much. Mentally though, as i aged i increased age threshold (also almost always found girl aged <20-23 underdeveloped physically). So i could only presume that if i'm aged 80+ physically i will still be attracted by women aged 25+ but mentally it will be a chore, so if i would have to choose i would take 40-80y old over 20-40 almost every time as a target of romance.
