Insurance companies and laws vary from state to state, but generally getting charged with being a Minor in Possession (MIP) will affect your car insurance rates. Car insurance for young drivers is already expensive due to teens being more likely to have an accident, so getting charged with a MIP will make the risk for the insurance company to cover the youth even higher.
Minor in Possession is what a person under the age of 21 in possession, ownership or control of an alcoholic beverage can be cited with by a law enforcement officer. The punishment for this citation varies from state to state but most offenders are fined and possibly given community service. Harsher penalties in certain states include revoking the driving privilege of the cited individual for a period of up to one year.
If you are found guilty of this violation in some states once you turn 21, and have not been cited for anymore alcohol related incidents, you may apply to the courts to have the conviction expunged. This may or may not help your insurance rates after that time.