I think this stems from "women should have / do it all" and a cost of living crisis
It's still a relatively recent phenomenon that a significant % of women go to university and have a career in the grand scheme of things. This delays having children and also gives women a choice - a choice because they can afford NOT to settle / to NOT have children if they so wish due to financial independence.
It's expensive to have children. Most educated people tend to agree it's best to be in a financially stable situation first. Married - expensive, home purchased - expensive, certain level in career to afford those two life stages - takes time and again delays having children.
I lived in China for 5 years. I asked female colleagues if they'd have more than one child. They said "no, it's too expensive and I wouldn't be able to give them everything. My parents gave me everything they could and I want to afford to give my child everything too. I would feel like my child would lose out if I have another'.
With delays in settling down, achieving financial stability, riding out a career, etc. people start TTC later and it's harder to fall pregnant as you age. This leads to issues with affording IVF or simply missing the boat, thereby reducing the number of children born. Some people end up with no children or fewer children because they unfortunately can't have them.
The age bracket is misleading, I'd be more interested to see how many % have kids aged 40. At 25 most people these days aren't thinking about having kids.
I have a friend who pursued a family as a solo parent with donor sperm in her 40s. She is exceptionally well off and has a strong career. She relies on her mother and nanny for support, she moved out of the city and capitalised on wfh post-pandemic.