Joining the police force on official servers will be largely automated. We anticipate that there will be some training tutorial and a quiz at the end that you will be required to complete in order to play as a cop. Private servers will have much more control over this, and can choose to white-list specific individuals rather than allow the automated system to decide. There is no age restriction on becoming a police officer, but some servers might want an adult rather than a teenager as a police officer. And you don't need to live in the US to become a police officer.