Respectfully, your question is confusing. Have you or have you not been charged with underage drinking? If you have, then the police must have had some, if not all, of the information you state they did not have. If you have not been charged, it is highly unlikely you will be charged after the fact. Although, you still can be charged if the officers have enough evidence to support the charge. The evidence could be from your testimony under oath that you were drinking and underage, or your friend's testimony that you were drinking and underage.
If the statute of limitations for charging you has passed, then you could not be charged with underage drinking. But, it is highly unlikely this is the case. In most states, the prosecution has years to charge you.
Bottom line: You should consult with an attorney in your area who is expereinced in these matters before testifying in your friend's case.