It's not like the data is hard to find. The >35 myth comes from data that is over 300 years old: https://www.bbc.com/news/magazine-24128176

This 2021 study suggests that fertility doesn't even begin to decline for women until age 37.1, assuming other factors (general health, nutrition) are equal: https://jamanetwork.com/journals/jama/article-abstract/2778126

A 2002 study found that 82% of women aged between 35 and 39 fell pregnant within a year: https://academic.oup.com/humrep/article/17/5/1399/845579 Among women aged 27-34, 86% conceived within a year of trying. The 82% figure for women aged 35-39 is not a significant decrease. And it doesn't just drop off at 40, there's a gradual decline. And that's just for conceiving 100% naturally!