What he's saying is that women are generally at their hottest and get the most attention from 18-30. They have the position of power in attracting a mate. If they have 20+ partners they clearly took advantage of that.

If you're a 40yr old man who has worked his ass off to reach a position where you are an attractive mate, you probably now have the position of power instead of a women of the same age.

The usually unsaid part of this is that the woman in question wouldn't have given you the time of day when you were in your 20s. It's only when you are in the position of power they start to consider you as an option, at which point it makes less sense for you to consider them an option.

That is why this feels like a slap in the face for a man, and also why men go after younger women. When they were a young man, they had no power. When they get older, they have more power.