I really don't think there is any evidence for this.

[–] VestalVirgin -1 points Edited

It is really hard to separate different factors.

High body fat percentage and absence of a stable father figure both are known factors for early puberty in girls, and, well ...

Parents who worry about healthy eating are also more likely to be committed to sheltering their daughters from porn. And obviously, a single mother who has to juggle job and children all on her own is going to have less time to worry about things like healthy eating, when it's hard to get her kids fed at all.

Besides, how many parents are going to admit that their daughter was exposed to porn if you ask them about it for a study?

You'd have to study adults and ask them whether they were exposed to porn before entering puberty, but to get a real comparison you'd have to know when the girl's mother entered puberty to rule out genetic factors.

Here's an article that lists domestic violence as something that can cause early menstruation: https://health.usnews.com/wellness/for-parents/articles/2017-04-06/why-are-girls-starting-puberty-earlier

I totally would consider being exposed to porn domestic violence. Even if the parents are not the ones doing it, it is going to cause a lot of stress to young girls.

(Regarding sexually exciting romance novels, I am more doubtful - girls are probably more interested in those when they already started early puberty ...)