It depends on the film, my 9 year old has seen 12a's over the years, but there's some I wouldn't let him watch. My 6 year came with us to see Thor last weekend, he loved it. Dr Who is far darker and scarier. The fighting was mainly gods and creatured thumping each other. There's a part where a creature gets a hole in it's head and you can see through, but it's like an empty shell, no blood, no brains, nothing squirts
As a family we really enjoyed it, humour, fast paced story, lots of gods thumping each other, nothing gory.
Not sure if I'd use it as a party for 8 year olds tho, for the reasons others have said. When we went it was nearly all young adults who went, or a few parents with what looked around 10 yrs +, it didn't seem like the kind of crowd who'd appreciate someone coming in with many excitable younger kids on a birthday treat. Or maybe that's because we went to see it the weekend it came out, and all the comic book fans had rushed out to see it :)
I would, however, have been happy to have taken my eldest at 8 with a couple of his best friends. I know already what their parents let them watch, and it's easier to have a converstion about whether they were happy, if they weren't, we'd watch something else.