There's no minimum age, and we have a pretty diverse crowd. People from about 12 to 46, from a number of different countries (I'm 27 and located in Eastern US). I look for maturity in the application and general attitude of the people applying to determine if I think they should be allowed on. I check for ban history in a few databases and match it to what the person put down on their application. I typically err on the side of allowing people on, and email them to ask them more about odd things. We utilize anti-grief plugins and block logging plugins, so if anyone turns out to be a bad player, they get banned and whatever they did gets fixed. We also make backups in case anything serious happens.