its a maturity thing really. from a site owner perspective, a lot might reach the conclusion that most 8 year olds won't fully respect the safety element.
As far as the pain element goes, chronoing down the guns or even investing in .50 cal equipment would be a good way to make sure young customers can still have fun. this is important because A. you don't want kids sitting out after 1 game because its too painful (less paint sales etc.) and B. they probably wouldn't want to come back if it was too much for them.
From a player perspective, If you find a site that allows your young child to play then just make sure that they fully understand the safety rules BEFORE going to the site and also that they are well prepared for the pain side of things. as mentioned, the vast majority of 8 year olds will find it a lot tougher than 12-16 year olds etc.