I quite liked the movie, the actress is very nice, I also saw some interviews, here it doesn't matter the age, she makes a good interpretation and maybe someone more mature and self-confident was needed for those scenes where she is not afraid to show her physique, which will be criticized by many.
The movie has flaws, but that reminded me of the American horror movies of the seventies, which many are now referents and icons of horror. For example The Texas Chainsaw Massacre has a lot of mistakes in its scenes, the low budget was noticeable, etc, but even so it still impresses and is a classic. Many other horror movies were a success despite having many mistakes, continuity flaws, crappy effects, etc. I prefer that kind of movies, they have a certain charm for me.
If I'm being honest I didn't like The Texas Chainsaw Massacre, it's one of my least favorite slashers. It's okay to enjoy some scenes, the most morbid ones in fact, but not the whole movie.
The age of the actress playing the role of a teenager may not matter, but it is quite noticeable for the plot. What I liked the most is the way they used this social problem is a good story with an undercurrent of revenge and guilt, where the heroine proves that she is not an evil person in reality.
You are right, many movies have mistakes and sometimes they are absurd, some of them make me laugh actually, but that doesn't stop me from enjoying the movie.