I was shooting a gun by 6, with a lot of help and supervision. I was handling a gun on my own by 10, still under supervision. (I never hunted, nor did my Dad. It was all just target practice and skeet shooting.) I wouldn't have a problem with my daughter attending this kind of party at age 8. Shooting targets is a lot of fun; I enjoyed it a lot when I was little. They specifically talked about teaching safety, always critical, so I see no issue with it.
That being said, I really think it has a lot to do with how you were raised and your own comfort level with guns. If you as a parent aren't comfortable or familiar with guns, I think it's going to have a lot of impact on how you feel about your child handling them. I would be perfectly comfortable and confident assisting my DD at this type if party (as I wouldn't leave her alone), but I can see that many would not. There's really no one right answer on this; it completely depends on the parents.