I had to watch this in school when I was about 12 or so. Despite probably being "old enough" I had nightmares consistently for a month. To this day, the acid trip of death and destruction scene that shows bunnies choking to death haunts me. I am now an adult and have seen scary movies, R rated movies, gore-y movies. Nothing has ever messed me up as much as Watership Down did.
It's important in the same ways that Animal Farm is, but that book is way easier to stomach.