They're past their sell by date, have far fewer opportunities, might not be feeling very well and are desperate not to be alone, so they pretend not to be abusive liars, cheats and completely selfish.
I remember the movie Cocoon, from many years ago, where the narrative was that a bunch of old people in a retirement home found mysterious cocoons in a swimming pool. When they swam with them, they started to get younger.
It was quite an interesting movie, but one dynamic was pitiful and sad. One old woman had tolerated her abusive, cheating husband for years (cheating is always abuse of course, see chumplady.com to debunk any disagreement on that) until he had been forced by old age and oncoming infirmity to behave like a normal man and treat her with a modicum of respect.
As he grew younger, he went straight back to being a filthy, pathetic, lecherous, pos to her great distress.
Obviously, it was just a fictional movie, but the premise was fair.
Once an abusive, cheating pig, always an abusive cheating pig. Given the opportunity, youth, vigour and chance of course they'd cheat again.