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.