The "different clock" hombre_fatal is referring to is the "biological clock" - A single childless woman aged 35 who wants a family with two or three children has very little time to waste (assuming she wants a traditional two-biological-parent family structure which is extremely common among members of the upper middle class).
They need someone ready to make a lifetime commitment - and as it's rare to marry without dating for at least a year or so, they've only got a few rolls of the dice left.
If that's not where hombre_fatal is in his life, such women won't see a future with him.
People in their early 20's are generally students, they have different priorities, lower incomes, different cultural differences etc... It seems easier to find 40yo who share my interests and my lifestyle in my experience.