Hell, if a six year old wants to play let him play. It's up to the parent or legal guardian. It's really not a big deal a little kid goes out with his family "Hides in the back" shoots someone, gets shot out, cries, and then laughs about it with their family. What age did you start playing paintball? If you asked everybody on the forums I bet %75 started before the age of 12, so if every field started at 12 %75 of the forums might not have started playing paintball. I know I started at 10 years old, so I might not have ever played paintball if the field owner of my parents thought I was too young to play paintball.
I would strongly disagree with that, as someone who has worked with kids that age for over a decade I know they don't pay enough attention to take part in a sport like paintball that requires them to strictly adhere to certain rules to ensure their safety. Refs have a hard enough time telling adults to keep their masks on, imagine a 6 year old who has a rental mask fog on him, he could potentially panick and remove his mask because of lack of understanding the danger. I would say a good age is 10 or 11, at this age children have had enough experiences with danger and personal injuries that they can remember. They can then fully realize the very real dangers of removing your goggles during a live game. To each their own, but I started at 11 and I'm glad I didn't start at 6 because I couldn't even draw in the lines at 6.