There are few, if any, advantages to underage drinking. It does, perhaps, give teens a way to socialize. You might argue that drinking when you are a teen helps you to grow up faster by learning how to handle adult things like drinking. But these are very flimsy claims.
There are many disadvantages to underage drinking. The most important of these is that it can lead to many health or social problems. When teens drink, they are more likely to become alcoholic than if they wait until they are older. There is the danger of drunk driving. Teens who drink are more likely to engage in dangerous sexual behavior. Sexual assaults increase as teens drink. All of these sorts of problems are the disadvantages of underage drinking.