My general observation for women is that its not so much biological as sociological.
When you're a young, attractive girl its easy to find young attractive guys to sleep with. Many girls I know date tradies who have the 'bod' but not much happening upstairs.
When you're an older woman looking to settle down the standard becomes higher - along with the 'bod' traits such as financial stability, reputation and being able to hold a prolonged conversation become more of a concern.
Suddenly, you're competing in a much smaller market of white collar men, their numbers further dwindle when you exclude those camp as a row of tents.
Tradies generally have bigger incomes earlier too.
Though a friend told me his 24 yr old daughter who has double degrees in Engineering is going to start out in the Pilbara/Perth two weeks on two weeks off gig at 120K opening salary!