The thing about fictional text, is:
a) it's sometimes a lot easier to mentally skip past things you're not ready for, and;
b) the level of description.
For example, here's a line from from of my favourite children's stories when I was young.
"And then the dragon ate the naughty prince all up". Fairly standard, I think. Now, think about all the bits that are left out of that line. If it was an adult horror novel about mythical creatures eating people alive, there'd be a lot more, erm, description about being eaten alive.
Now, imagine the scene, being shown visually, and the level of detail. The equivalent of the children's book would be more of a funny cartoon. The naughty prince would be swallowed in a big, comical gulp with no gore or screams. Then the friendly dragon would give a big burp.
The 18 plus version would have screams, rending and tearing, and close up special effects of the teeth coming towards him, etc.
A 12A film would be somewhere in between.
I loved the Hobbit before 12, but there were lots of things that then I would not have been able to watch in too realistic detail.