When I was 7 years old.
Aww, Cowboy... when you were seven years old, the original Frankenstein movie hadn't even been made yet!
Maybe you meant the book.
I don´t think I ever watched it yet but my mom allowed me watch other scary movies when I was 11/12 when I was with her.
Allow? I had a TV in my bedroom and watched whatever I wanted to. This of course was before cable tv. I have to admit, some of those Chiller Theater and Creature Feature movies gave me nightmares. The ones that bothered me were: