Are there any studies how much of this is caused by hormone levels and medical infertility? ....and how much is caused by people studying until 25 and working on their careers until 35 before they even star trying (compared to going to highschool until 18, starting a job at 19 and making kids at 20)? Also, how many people can actually afford to move to a place with an extra room for a child + all the extra expenses when you actually get a child?
I know it's anecdotal, but i know very few couples who want a kid, but are unable to get one while trying... i know more than that people, who didn't want a kid, but it "just happened", and i know A LOT more people, who'd like a kid, but "when they can afford one", and that 'when' isn't 'now' (mostly due to housing prices)
edit: also looking at a whole population, there is a huge difference if you start making kids at 20 (since you have time to raise 2, 3, even more... if you can afford them) or if you start making your first at 35, where the biological clock is showing it's signs, and even a second kid is a big "if", if you don't start immediately after the first one.