The book, The Exorcist, was a big hit but the movie version had little support in Hollywood so at the end of the day it ended up with a director who wasn't well known and constant rejections by potential actors. So they ended up with actors who were little known or not known at all as in the case of Linda Blair (age 14), who ended up with a nomination for an Academy Award and won the Golden Globe Award for Best Actress.
It was a shocking film in 1973 with all kinds of audience reactions including gagging, throwing up, and running out of theaters. Warnings for pregnant women began showing up which is also something they did with the movie: "Omen." People began standing in long lines as it became a cult classic; a right of passage for young people.
I was ten years old when the movie came out and I don't have a clue how old I was when I saw it. I just remember being absolutely scared to death and encountering people in my life who I thought needed to be exorcized. :)