There are a lot of factors involved in fertility, and it’s difficult to do any real research as opposed to just crunching numbers using vital-stats data.
I had my first baby at 24 and my 10th at 45. Historically, this is not that unusual, although societal nutrition levels have affected the average fertile years in certain times and places.
Many of the moms I knew (when I was raising children) did the same: They started having children in their 20’s and continued into their 40’s. Other moms started in their 30’s and continued into their late 40’s. And a few others started in their 40’s. None of them used fertility doctors. Most were surprised by their pregnancies.
I had my first 3 kids at ages 28, 31 & 34. Then I had 4 miscarriages, the first at age 37 & then 3 in 13 months when I was 41/42. So I could get pregnant easily but keeping it was the problem. Of course, I have a uterine abnormality (incomplete uterine septum & heart-shaped uterus), so I’m lucky I was able to carry my first 3 babies to term in the first place!