AN AMERICAN teen-ager has become a well-known figure and the center of controversy in Europe. She is not a rock star or a world-class athlete, but a self-confessed murderer. According to Newsweek magazine, Paula Cooper, who celebrated her 18th birthday in the Indiana Women's Prison last month, has become the focus of the anti-capital-punishment movements in Italy, France, Spain and West Germany.

Miss Cooper is hardly a heroine. When she was 15 years old, she and three friends hustled their way into the home of Ruth Pelke, a 78-year-old Bible studies teacher, and robbed and killed her. Paula Cooper confessed to stabbing the victim 33 times. People in Indiana who remember this vicious crime bristle when millions in Europe sign petitions demanding a pardon and others refer to the teen-ager as the real victim in this case. But while few Americans would agree with this characterization, many share the Europeans' abhorrence of capital punishment for teen-agers.

Since the death penalty was reinstated in this country in 1976, dozens of young people have been sentenced to death for crimes committed before they were 18. Some of them have already been executed. Only six of the 35 states that have capital punishment prohibit its use as a penalty for crimes committed by minors. A handful of others require that age be taken into consideration at the time of sentencing, but in most jurisdictions this critical fact is not thought to be particularly important.

None of the democratic countries of Western Europe allows capital punishment, so it is easy to understand why people in those countries find the practice of executing teen-agers particularly repellent. You may believe that American laws are not their business, or you may suspect that many of Paula Cooper's supporters are generally anti-American in a number of areas. But the point being made is nevertheless valid. No civilized society sets the same standards for children as for adults.

Youngsters can do terrible things -- as Paula Cooper did -- but they are not generally penalized as severely as mature persons who break the law. Even those who do not share our view that capital punishment is always wrong should be horrified by the prospect of using it in the case of someone not considered mature enough to sign a contract, drive a car or vot