Underage drinking is not a crime in WI, it is a civil forfeiture. When the prosecutor indicated this would go on your record, s/he likely meant one of two things:
1. This violation is on CCAP. If you were ticketed by a state trooper or county deputy, your ticket is likely listed under your name on CCAP, Wisconsin's circuit court access system. You can look yourself up on CCAP at: http://wcca.wicourts.gov/index.xsl. If you were ticketed by a local police officer from a city, town, village, etc., this violation will not be on CCAP. The easiest way to tell who arrested you is to look at your ticket. Towards the bottom, there is a line for the arresting officer's name. This will usually indicate if the officer is a trooper, deputy, etc.
2. An underage drinking citation WILL go on your driving record. (I recently confirmed this with a representative of the WI Department of Transportation.) This way, if you are ticketed for underage drinking again, the ticketing officer will know this is a second or greater offense. The fines and other penalties go up substantially for each underage drinking citation you receive.
AND BE CAREFUL: Even though underage drinking is a civil charge/fine, the court may decide to suspend your driver license. For a first underage drinking violation, if the court decides to suspend your driver license, the suspension is at least 30 days and can be for up to 90 days. See Sections 125.07(4)(b), 125.07(4)(bs), and 343.30(6)(b) of the Wisconsin Statutes.
Further, there are certain circumstances in which the court MUST suspend your driver license. These scenarios, however, do not occur unless you have more than one underage drinking violation in the past 12 months; and you were driving a vehicle at the time of the ticket.
Hope that helps, and take care!