If it's secluded, private property, then it's usually cool. The main thing is just to keep it out of the public eye where a passerby could mistake what he sees and call the police. If you have neighbours that use their land, it's probably best to warn them you're going to have a game so they know what's going on and don't call the cops. And if the land does border public land, then it's also a good idea to warn the local police department ahead of time of what will be going on so if they DO get calls, they don't send SWAT to join in the fun.
As for under 18, usually parents have to be there, and often, also be in the game. It really depends on the game host. His game, his land (or borrowed land), his rules. Realy though, unless it's a private game you set up, the likeliness of you playing before 18 is slim.