Would you mind sharing some of these evidence ?
In a place like the US you can get more granular data down to the county level and see the same thing. Economic centers having the same trends in first birth age as other developed nations.
There is an even distribution of sexual activity, you might just have to trust me on that one, but there might be some study to cross reference and show thats a constant.
Given the same result occurring in so many places, the evidence suggests people aren't interested in children until they are afraid they can't pass on their genes in a healthy way. Fix that and I would expect to see the age increasing another ten years. Fix the stamina deterioration and I would expect to see the age increasing further again.
"People being more selfish" might definitely be a factor, but you might come across as considering it like the factor, which would required exceptional data..
I used the word suggest and chose it specifically. I did consider those things and chose not to write a dissertation to convey the exact same concept as the word "suggest". Do with it what you will.
It is possible to be reductive because the same thing happens in so many places. I think there are very few exceptions to "educated, choices, access to contraception" where "cultural peer pressure to have children young" actually works.