Grave of the Fireflies isn’t about the nukes its about the firebombings and the part with fiction vs reality means that they don’t understand that this was a long time ago. As said its appropriate for 12+ in my opinion. But below its just not.

Just because a rating agency says something doesn’t mean its appropriate, they have strict guidelines, these guidelines revolve around obscenitys, violence, drugs and similar, they don’t go on “may not be appropriate for children because the story itself is dark and sad” 12+ would be a ok rating in my opinion, again, its not meant for children, German fairytales aren’t either, even though they are classified differently.

Kids below a certain age cant even comprehend the story.

No, you shouldn’t traumatize your KID, so anything below 14, its absolutely ok to show the movie to a young adult or teenager. A kid does understand what suffering is, but doesn’t understand the difference between fiction and reality very good.

Thats the same argument as with German fairytales, they aren’t made for kids, they are for teenagers and above.

No you should not.