The min. age at the field I help to organise is set at 16. At 16 the players must first have a parent sign off on their playing with the group and after that for them to attend games without their parents they require another player to "sponsor" them and take responsability for them. We find out very quickly if we do or don't want them to be a part of our club, yet there are some younger players we'd love to see at regular games. And those few are quickly sponsored by a veteran player. Should any problems become of the sponsored player it's the job of the sponsor to deal with the problem. Should they refuse then obviously the player in question gets black flagged out of the club.
As soon as players are 18 they have the same right as every other non sponsored player.