Most countries around the world have set their drinking age limit below 21. Some countries do not even have drinking age laws while others completely prohibit alcohol consumption. This creates a different culture around alcohol than you might have grown up with in the US.
Alcohol is generally much more accessible in other countries and people begin to develop and establish drinking habits at younger ages than in the US. For example, sixteen year-olds in Germany are allowed to consume beer.
Take some time to research the culture of alcohol before beginning your study abroad experience.
If you choose to drink while you are abroad, keep in mind that country's customs and practices. Your program director and/or host family are great resources to ask about drinking customs in your study abroad country.