Grave of the Fireflies
It's Schindler's List in a few important ways. 1) it's about WWII, 2) It's a gut punch, and 3) It's one of the greatest movies you only need to see once, and likely will only want to watch once.
I've told the kids they can only watch that movie once they're old enough to drink.
If there was, wouldn't you want to know?
So, 6 months old? (When I grew up, whiskey was a common cure for toothache, gripe, or any other bother infants had...)