Minor in Possession (MIP) or Possession of Alcohol Under Legal Age (PAULA) is normally a misdemeanor (visit this site: http://www.fresnolawmen.com/felonies-misdemeanors-infractions-different-degrees-of-crime/ for explanation on different degrees of crimes) that applies in the circumstances of any under 21 person that is caught, seen or there is sufficient evidence of been drinking or even carrying alcoholic beverage. Though initially the law was created to prevent minors from the possession of alcoholic beverage, it also applies to drugs.
Since legislation concerning to alcohol is a state responsibility, each state treat differently the exceptions for this law. In Texas, for example, it is allowed that minors consume alcoholic drink since under supervision of his/hers legal guardian or parent, which is not on Mississippi’s legislation, and in California, the law only prohibits the public possession of alcohol while in North Carolina and in Ohio the laws have special recommendations for allowing minors to consume alcohol while in religious ceremonies.
Punishment for MIPs also varies drastically from state to state. In Texas, it is a minor crime that requires payment up to $500.00, while in North Carolina, for example, misdemeanors may be punished according to the class of the crime from 10 days of community service with a fine of $200.00 up to 45 days of service with a variable fine to be defined by the judge and in the state of New York, the fine is no more the $50.00. Minors caught driving and drinking may also have their driving license arrested and the determined fines may be increased.
Also, the law does apply not only for the possessor of the alcohol itself but for commercial places that sell beverages to minors, and for adults that offer or deliver alcohol to minors, with punishments that also vary.
This site has lots of information on this type of infraction.