Thank you for providing studies, this is exactly why I stated it with a lack of surety. The human part of the reproductive physiology class was a very small part, I'd say we learned more in detail about marsupial reproduction than human pregnancy. But also, both of your second links do state that the first birth is the most important factor, the second one stating that the protection of differentiated cells is by far the most important protection mechanism. None of the studies you linked talked about early sex steroid exposure, the first one only looked at taking hormones post-menopause.

I did state that having a child later wouldn't protect you if you already are on track to get breast cancer, and I was right-ish with that. People have more genetic damage when they're older, and the stem cells copy that damage when they differentiate, leading to an increased risk of cancer. That's what makes a late first birth more of a risk and basically what I was getting at.

You gotta keep it simple when you only have someone's attention span for 30 seconds, which is usually the case on the internet. Most people aren't going to go read scientific studies when not on r/science (though it's nice that they're open access, appreciate that).