I have an underage drinking offense in Portage County, WI.  This
offense is not posted on the wiaa circuit court record site at
http://wcca.wicourts.gov.  Nor is it a criminal offense.  The fine I
paid for this violation was considered a civil forfeiture. 
Technically, on a medical school application, do I have to answer yes
to the question "Have you ever been charged with or convicted of any
legal offense (other than minor traffic offenses, such as parking
tickets) including offenses where the records have been sealed or
expunged?"

You should probably look into the permanent sealing of your juvenile
criminal records.  That way, this won't be an issue.