It is disingenuous to say that ads NEVER encourage underage drinking, so one must say that sometimes, ads do encourage underage drinking. An ad could target certain age groups to make drinking seem appealing to them. If an ad were to target kids in some way, some kids would have the desire to drink alcohol because of said ad. Ads for alcohol in general, however, do not encourage underage drinking unless an ad is targeted at that segment of the population. Even so, one or two underage individuals may be swayed to drink by an ad no matter which demographic it targets.
Advertisements are meant to encourage everyone to purchase and consume a product. Individuals who are underage are in no way immune to an advertiser's persuasive ads showing alcoholic beverages. They need much less encouragement because they are at the age when it's common to rebel against parental and societal restrictions anyway.
Alcohol ads are no more influential in getting minors to drink than toy company ads are in influencing grown men to ride Pink Bicycles! The real culprit is the stigma and reputation of alcohol that has been and will be passed down through the years that says alcohol is the ultimate experience that everyone says it is.
I do not believe ads for alcohol encourage underage drinking. I think society, culture, and peers encourage underage drinking. Advertising is annoying and I would love to see less of it, but the American public also needs to be educated about its persuasive and manipulative methods. We don't need advertising but attacking for the wrong reasons won't make it go away.