While the law opposes underage drinking—or any kind of drinking in a public place—it seems to me that the bigger problem is not so much the law, as the culture.
I live in London, travel to mainland Europe very frequently, and visit the United States every so often. Each time I visit, I am struck by the influence the Puritans and prohibition have had on US attitudes to alcohol consumption, and especially the attitudes of parents to their childrens' drinking habits.
First, some notes on the attitudes to drinking alcohol in Europe. For those who have never visited any Mediterranean countries, alcohol, mostly in the form of wine, is a normal and expected part of family life, as well as pretty well every social occasion. In Italy, Spain, Portugal, France and many other countries, there will be wine on the table with every meal. I have sat in a restaurant in France and watched the owner eat a plate of seafood and drink most of a bottle of wine for breakfast. This is not particularly unusual in wine-growing areas. Children of ten years old and below will routinely have a glass of watered-down wine with the evening meal.
In France, your waiter will be surprised if you do not have at least a small carafe of house wine with your lunch. In Italy, it would be almost unthinkable to go to a restaurant and fail to order some wine with your evening meal, or a grappa with your coffee. In Spain, the evening meal is taken around 10 pm, or later, and again, there will almost always be wine on the table. It might not be the finest of varietals, but it will be dark, and dry and alcoholic. In a restaurant or bar anyone who looks more or less adult can order wine or beer, no matter what their passport or ID card might say.
In Germany or Belgium the drink might be beer rather than wine, but even at business events, many people will often drink either a beer or a glass or two of wine with their lunch, and most will have some alcohol with their evening meal. Even in the UK, habits are changing. Once it was normal to drink a cup of tea with the evening meal, but beer or wine is now more common. The point of this explication is that most children will have grown up with alcohol on the table and will have tried some—almost certainly with the blessing and encouragement of their parents—by the time they are 14 or 15, and in many cases well before that.
Drinking in public places is not banned in most parts of Europe, it is again, just a part of life. Cafes have outside tables. When eating a picnic in the countryside, or in a public park, it is completely normal and socially acceptable to have a bottle of wine or a few beers with the food.
Habits in the USA
As a point of law, in most parts of the USA, it is illegal for people under the age of 21 to either buy or consume alcohol in any place at any time. It is illegal for anyone (no matter how old) to consume alcohol in a public place. These rules appear to be rigorously observed by a majority of the population, even within the home.
Maybe I am not the right person to write this, and I know it is not a definitive survey, but based on the write-ups above and elsewhere, and my own experiences, I am going to discuss two young Americans. One aged 18 and living with his parents. His sister, just a few months from her 21st birthday, lives away from home in a college environment.
Both those people have access to money and both drink pretty much as they like. The young man—old enough to drive, old enough to be a father, and old enough to die for his country—hides the fact from his parents. The parents are completely unaware of the extent and regularity of his drinking habits. The young woman can talk openly to her parents about how much she drinks at college, but chooses not to consume alcohol in their presence. To do so would be seen as an outright challenge to their authority and expectations.
The parents are reasonably comfortable in economic terms. The mother has a European heritage, the father comes from old Episcopalian stock, but both are fairly liberal in many of their views and ideas.
Their son, who turned 18 in February, is in his final year of high school. Since his birthday, he has become an independent adult in the eyes of the law, and his parents now allow him a huge amount of freedom in terms of the hours he keeps and his social activities. But they will not keep beer in the house after an incident in which a few bottles went missing. In his den, there is a line of empty Smirnoff Ice bottles. His mother thinks it is a kind of root beer. His father once accidentally opened an e-mail intended for the son 'I have a bottle of Bacardi, are you coming over'? He thinks the friend who sent it is a bad influence.
Their daughter is at college three hours' drive away. She shares an apartment with three other young women. In a restaurant with her parents, she will not take a glass of wine with the meal, and her parents approve of her decision. "It's the law" they say. The four women enjoy their social life. One of them is president of her Sorority this year, and is responsible for the finances and the social events, many of which include the sale and consumption of alcohol. She, too, is 20 years old. It is unusual for any of these women, they say, to consume alcohol when their parents are nearby. Not especially for fear of being discovered, more out of some kind of respect for their parents' feelings.
As a European I find this very strange. It seems entirely wrong that the law and the culture expects a young person to change from a teetotal 20-year-old into a responsible drinker at age 21. Surely it is easier and more humane to introduce a young adult to the pleasures and risks of alcohol within the home, under good control, than it is to force him to discover it through binge drinking with a group of similarly inexperienced friends?
And if alcohol is a normal part of life, then growing children learn to use it responsibly rather than go off on illicit, irresponsible sessions, hidden from all public view. What happens then, when something goes wrong?
So, while the law may be in place, as it is in many European countries, it is more the culture which governs the use of alcohol in the home. It is this legacy of the Puritans, and its influence on alcohol—and incidentally sex—as being "Bad Things" which seems to me more of the problem than the actual law.