My two cents is that the leader is being a bit aggressive. After all, it's your field, and if he doesn't like your rules, he can go play elsewhere.
Even so, I think you should keep your options open, especially to younger people. Yes, 10 year olds may be pushing the bar a bit, but for slightly older adolescents, maybe you could help mentor them. Initiate a boot camp or something similar so that they can learn in a controlled, safe environment where they can demonstrate their abilities and maturity before moving on to 'free play' with the big boys. :P
I'm sure that even if you didn't allow younger people, others would still come. For example, the age limit here in Calgary is 16+, with occasional 13+ 'noob games' where the less experienced can come and learn the game respectfully.
Also, criticize me if you may, but I believe 5 acres of field is plenty enough for Snipers. Or have I misinterpreted your information?
Anyway, just my opinion on this. Still your overall decision.