So Carrie got her period at 17 or 18?

Chloe was actually 15 in this movie. They filmed this movie way back in 2012 and released it in 2013. She just turned 18 this year.

That a girl has her first period at 17-18 is just marginally believable. That a girl at that age does not know what a period is, however, is totally ridiculous today. Even in the original film set in 1976, it was straining credulity a little too far. Perhaps in that era, you had to borrow books from the library to learn about the human body, and Carrie's mother certainly would not have told her. But in this Age of Internet, that is simply ridiculous.

This is one problem which remakes have to face - in this case rather unsuccessfully. In this remake, the scriptwriter did try to "modernize" the story by introducing Internet bullying. But strangely, no one seemed to be aware that our dear Carrie, just by typing one or two words in a search, would get something like 1,001 pictures of female (or male) anatomy and as many YouTube videos on female health issues, birth of babies and things like that. In the film, Carrie searched the Internet for telekinesis. Unless she was retarded or totally devoid of curiosity, she would certainly have searched long ago for information about her own body.

Also, isn't biology a subject in secondary school? Perhaps back in 1976 there were still a few girls who believed storks bring babies and kissing gets you pregnant, but today there is always some form of sex education fairly early in the schools. Depending where you study, it is also likely there will be some counselling about prevention of unwanted pregnancy, and some schools even distribute prophylactics to the students. 

I suspended disbelief in the original so it's easy to do so again here, Carrie looks much younger in this one than in the other film though which makes it easy to digest.

https://www.youtube.com/watch?v=46dsSv1SzOI (Carrie in 4 Minutes)

Both these questions are answered in the original movie.

I just asked my missus when she started her periods and she said 17. She did look at me like I had 2 heads when I asked but yeah, she was a late bloomer.