In the film it says that Marissa killed her mother 15 years ago. It has a flashback scene of them driving in the car and Hanna is obviously a baby of about a year old in it. I saw the film two times and I remember that because I really liked that scene. 
 Those aren't proof, those's are just short reviews and synposis of the film. You want the real proof? Go watch the film again. 
 It is coming out soon on DVD/Blu-Ray in the UK and I have pre-ordered it and I will re-watch the film and come back. 

 [] Have a look there. It says Hanna's age is 14, but the script had some re-writes until being shot and edited and is based in 2010-2011. 

 It says in the flashback scene, taking place in 1995 "A man, a younger ERIK HELLER crawls out. He is holding a two year old child in his hands." If she is two years old she was born in 1993. If anything you should be looking for actual proof for the year it is set.