In most Swedish municipalities (Kommuner) it is strictly forbidden to drink any alcohol in public. Getting caught could lead to a fine of 500 SEK (ca. 50 Euro). But in many cases the police confiscates your drink and simply pours it out.
Sweden is pretty strict when it comes to accessing alcohol. One has to be at the legal age of 18 to be allowed drinking any alcohol. Buying beer or wine in a restaurant, pub or night club in Sweden one has to be 18, too; same if you want buy a low percentage beer/cider (<3.5%) in a supermarket.
Shopping at Systembolaget often leads to the cashier requesting to see your ID to verify your age – unless you look older than, say, thirty. The cashiers have to check everyones’ ID if they believe that a customer could possibly be age 20 or lower. So, yes, if you’re older than that and you get asked to show your ID, you may take it as a compliment 😎
Systembolaget is a government owned chain of alcohol stores in Sweden. It is the ONLY retail store allowed to sell alcoholic beverages that contain more than 3.5% alcohol. The main motive is to minimize alcohol-related problems by selling alcohol in a responsible way, without any profit motive.