Women who go for age gap relationships usually want a more dominant daddy figure. However, with age the man will become weaker and weaker. When he's 60 and she's 10+ years younger she'll feel like she has to baby him. It's a turn off.

In same-aged relationships, the opposite happens. The woman usually starts being in a more dominant position but gradually loses power because a 50 yo man (assuming he's not an obese bum) is usually way more attractive than a 50 yo woman. Women tend to look older after having kids but also I remember reading a study that explained that men tend to produce more collagen and thus tend to look younger than women.

Every single couple I know in their 50s, where the man is at least somewhat attractive, the man is a cheater. On the other hand, women who have affairs are usually married to older men or very weak men.

Of course there are other reasons why age gap relationships fail, but I feel this is the main one. The old man becomes weak. Everything else is secondary since we know that if a woman is truly in love she will move mountains to make the relationship work.