Well, as a new player, I don't have any experience on the fields, I feel like I still can add my two cents.
I'm 19 years old and my brother is 15. He is the kind of guy that I would allow on the field even if he's under the age most people find acceptable. He can and will be mature with his weapons, will follow the rule and be a member that can help a community grow.
I live with my brother since he's born, and I saw him evolve with weapons for a few years. THAT is the reason why I can judge he's able to get along in a field.
The point is. Yes, sometimes younger people can be mature and everything. But I personally think it's safe to assume he's NOT mature enough and put an age cap, which for me would be 17 or 18. It's not because my own brother is mature enough to play airsoft safely, that I think he should be allowed to the field. The weapons are, here, in nearly any shops, and everywhere online 18+. And I agree with that.
Kids should be taught how to handle a weapon, how to carry it and how to safely use it way before he's 18. But it should be via a mentor (in the case of my brother, me), and they should not be allowed to POSSESS a weapon themselves, but rather use the one of an older friend or people that they know.
If you read everything, thank you. That's simply my opinion, feel free to think I'm wrong.