I was/am an OW - currently exiting a non-relationship. The reason he was "interested" in me is because he liked the attention I think. I absolutely worshiped him - predominantly I suspect because I have low self esteem and he has a streak of narcissism so I responded strongly to his controlling character. I am single so it's easier to provide 100% attention and worship in a new relationship that than if you are dealing with the normal realities of life with 3 kids etc with someone you have been married to for years.
An affair is a warped fantasy not grounded in real life.
I don't think what someone looks like etc really is what it's about. It's how the OW makes a man feel. Eg misguided angel's comment above
"She's not as attractive as you, she's not as intelligent as you, she's not as interesting as you ... but she really needs me".
Sounds as if he liked feeling needed. Sometimes being with someone ridiculously young makes a man feel younger etc.
In my case, as it happens I am younger than his wife by about 15 years and when the affair began I was considerably more attractive and living a very glamorous lifestyle. Someone looking at me externally would never guess I have such chronically low self esteem as my life looks great from the outside. But none of that was what it was about.
I suspect if his wife found out about me (saw me/met me/googled me) she would think it was all about a glossy package stereotypical younger mistress but it definitely wasn't. He just liked being the center of my attention - actually not just my attention, the center of my universe. How I feel about him is really pathetic as he was very emotionally abusive.
I am aware of several other OW he has had sex with in a two/three night stand type of way and none of them were particularly attractive.
Focus on appearance is not what an affair is about IME. If a MW finds out about an affair, external information is all she has and comparing what some looks like/age/ etc is natural because that's the only information going.
Really what it's about it going on in the head of the MM and how he perceives the OW and how he feels about himself when he's with her.
It's pointless comparing/trying to compete with it because it is all unreal. It's a sick sort of fantasy of a perfect relationship for a man - a woman who adores him, is always ready for sex, never argues with him as time together is so limited, always pathetically grateful for a moment of his time. It's not sustainable because it's all built on nothing and real life isn't like that. Which is probably why so many relationships that start as an affair fail.