I find that there are really 2 questions being asked here:
1) WHY do teenagers drink alcohol?
2) Will this necessarily mean they will develop a drinking problem in later life?
Many teenagers I see drinking alcohol illegally, seem to do so through boredom rather than a desire to rebel against their parents. This is particularly so in small towns, where they may not be able to find anything to do but stand outside shops or on the street, slugging from bottles of cheap cider. I am sure if I asked them whether they ENJOYED the taste of alcohol, many of them would say no. Personally, my first taste of alcohol was not pleasant (wine with a meal when I was 17), and for a long while I preferred to either not drink, or drink small quantities of alcohol with a mixer.
I feel that teenagers who do drink alcohol run the risk of developing a drinking problem later on in life, although this is not always the case. Even so, it is not a good idea to encourage teenage drinking, considering the serious harm it can do the organs within the body at such a young age. Binge-drinking as an adult is known to harm the liver irreversibly and be a main contributory factor towards cancer and other life-threatening diseases. Imagine what it would be doing to a child.
The reason why we have a law in this country forbidding our youngsters from drinking is simple: it is a matter of looking out for THEIR health. It is considered that a child becomes capable of making adult decisions by the age of 18. Before that, in the eyes of the law, they are considered a minor. Even if they think they know what is best for them, this is not always the case. A child, or even a teenager, is incapable of moderating how much they are drinking by considering the damage it may be doing to them inside.
In summary, regardless of the reasons as to why teenagers would drink (and I must add, it is only a minority of teens who DO choose to drink, the majority appear sensible when it comes to alcohol), it is a bad idea on so many levels. It could only take 1 more alcoholic drink to mean the difference between a child being drunk and a child suffering the effects of alcohol poisoning.
I know if I had a young teenager, I certainly would not wish to risk my child becoming another sad statistic by encouraging, or turning a blind eye, to their drinking habits. Alcohol is to be enjoyed by adults, and that is the way it needs to stay.