I think there are a number of factors, and a few of them are hard to pin down.
In my experience, it's that there is an attitude from a very young age that women's bodies are inherently sexual and that women's bodies are objectified. There's just a general lack of respect for women. In my experience, I've seen this since a very young age. I know that it's in grade 4, grade 5—around puberty—that boys start to make comments about women's bodies and also start to view pornography.
I think that generally there's an attitude that is perpetuated in the media and I think also sometimes through adults. There are often, especially in school communities, adults and teachers who often don't call out this behaviour that happens. I've had experiences of a comment being made about my body—and this was a while ago, so not at my current school—and often teachers would not actually call out that child and explain why their comment is offensive or wrong. They won't explain the roots of it or actually take the time to talk about that.
I think those are two factors. A big thing is the media, obviously, and how they perpetuate this image of women and young girls.