Bottom line is that the age thing is not a "society" thing. It's a personal preference. In fact, even Benjamin Franklin wrote about the merits of dating an older woman.
It's obvious you find it distasteful, so leave it at that. But just because YOU find it objectionable, don't impress that society does also.
"I think the cause is that she is afraid of getting old. And she feared spending the rest of her life with me, with the anchor of our creaky old house, our bills, our kids, our pets, and our dull middle-aged lifestyle dragging her down, down, down."
Alot of this is speculation on your part. Bottom line is that you'll find out that all of the analyzing you do won't help your situation. They are your W's decisions and for whatever reason, she felt unhappy. It will be up to her to decide ultimately what will make her happy. If you can't live with that being out of your hands then you might as well file yourself and move on.
Right now you do have alot of resentment built up in you. If your W were to come back now, I have a feeling that you're going to be very judgemental of her and that would drive her away again. Sometimes people just have to do what they feel they have to. You have that right and so does your W. It may not make it "right" in your eyes, but it's your W's perception that matters to her.
If you can find a way to sympathize with her, it'll help you with forgiveness.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.