It isn’t about “the wall” in terms of appearance; it is very specifically about bearing children.
All the issues are tied together.
1. They’re not as physically attractive as when they were younger
2. you have much less choice in terms of when you will have kids, how many, what the chances are of miscarriages and birth defects, etc etc.
3. If you knew her in her 20s and she thought you not good enough, how can you possibly think that in her mind she is doing anything other than settling if she warms up to you in her 30s? Are you prepared to accept being settled for....being the schlub who just happened to be available when she ran out of other options? Don’t you have any self respect? How committed is she really going to be considering she didn’t really want to be with you in the first place?
For all of the above 20s, not 30s.