(NSFW?) Why are girls that have their periods young seen as sluts?

I remember when a girl, Becca, had her period at the age of twelve. Nearly all the people in my class, namely the boys, called her a slut, a whore and dirty. Granted the atmosphere in my class was never what you call ‘healthy’ but I heard from other people that they know people who were also called names for getting their periods young.

I was wondering if this is a sexist culture thing and if it’s happening or has happened elsewhere. I find it quite peculiar that some associate something as natural as periods and age with the negative.