There are many threads answering this question.
If memory serves right, I remember someone saying that if you're not mature enough to use the search function, you aren't mature enough to play airsoft.
It's all about the liability. If you're underaged, you cannot be held responsible for your actions. You legally are not an adult.
I personally don't like the whole idea of parents signing waivers then dropping their kids off at a field. That's paintball. This is airsoft, not a daycare.
Use the search function but basically, it's all about liability.