I think age is pointless. I know plenty of guys 20+ who I wouldn't be caught dead letting them come to an airsoft game and vice versa lots of guys 13 and 14 mature way past their age.
If I'm playing I say anyone who wants can come out and their conduct determines whether or not they get invited out again.
Setting strict guidelines based upon numbers is really silly in my opinion.
The games I normally play in though are more informal, a buddy of mine has an acerage with a decent amount of land up north and we usually will play there, those who come out are normally people we've met here and there, friends who've expressed an interest and have been invited out. Not too often will we have people come out that we don't know at all.