TEEN pregnancy, teen contraception, teen delinquency. With the increasing prominence of these issues, you'd think that teenagers in recent years were getting into trouble like never before.

Funny thing is, they are getting into trouble like never before. The reason is pretty simple. People are teen-agers for longer than they used to be. In the past century, the gap between puberty and marriage has expanded enormously. "Teen-agerness" is not confined to the ages 13 through 19 anymore, but to the expanding period when the desires of adults coexist with the responsibilities of children.

Until 1960, the declines in the ages of puberty and marriage roughly mirrored one another; the length of the teen years did not increase. From 1850 to 1950, the average age of the onset of puberty declined steadily, from 16 to 13, for girls. Pubescence among boys has consistently begun about a year later.

From 1900 until 1960, the average age of marriage similarly declined, from 26 to 23 for men, 22 to 20 for women. Since 1960, however, these ages have shot up again, so that we have just about returned to the 1900 marriage ages of 26 and 23.

WHAT all this amounts to is that teen-agers are, well, "teen" for much longer than before. That state, the gap between puberty and marriage, used to be about 6 years for women, 9 for men. Now, those figures are closer to 10 and 12. Today's teen-agers have to spend 3 or 4 more biological years than their grandparents in the nebulous limbo between childhood and adulthood.

Somehow, this obvious biological explanation for the explosion of teen problems has escaped the purview of social critics. Religious moralists would have us believe that the problems of pre-marital sex, teen pregnancy, etc. stem from the ills of 20th century society and the breakdown of American morality. Some even confuse cause and effect, emphasizing the pernicious influence of rock music (this means you, Tipper).

Their slightly more scientific brethren, known as sociologists, would pin the blame instead on broken homes, bad education, two-career families, and the premature influence of the sex-filled mass media. The diagnoses of both sociologists and moralists imply that we need to rebuild teens' social environments to condition young adults better. This is not a constructive recommendation, because it rests on the assumption of moral or social decay. Claiming that things were better in the good old days gets us nowhere.

THE insight that biology has greatly exacerbated the complications of the teen-age years indicates that our social system and our anatomies have been working at cross purposes. Although children are maturing earlier and earlier, we are detaining them in high school as long as ever. As a result, people who are ready and able to assume adult lives continue to be treated as children.

One hundred years ago most 16 year-olds were too immature even to consider a full-time job or marriage. Now they are physiologically almost ready for these responsibilities, but are still stuck in the high schools they have outgrown. No wonder they listen to Motley Crue. Our educational system has become perversely destructive; that which was designed to foster growth is stunting it instead.

The obvious solution is to condense secondary school by two or three years so that students can finish earlier. Shortening school would make graduation and the physiological onset of adulthood roughly contemporaneous, as they ought to be.

This broad social measure would force teens to support themselves at an earlier age, encouraging them to marry younger. Teens' mental growth would be in closer harmony with their physical growth, so that "education" would not hinder the maturation of teen-agers into adults.

Like any other, this reform would entail some complications. School curricula would have to be condensed, which would demand a major overhaul of our educational system. An even more serious problem would arise as young adults entered the workforce earlier, causing a large rise in unemployment.

These, however, are short-term problems. Condensing and improving school curricula would be a long-term solution to increasingly pressing social problems. Granted, any exercise in social engineering carries with it certain risks and costs. But the alternative solutions to today's teen problems, which emphasize the inculcation of morality and responsibility, rely on abstractions that are even less subject to human control and, in any case, do not deal directly with the issues at hand.

Allowing teen-agers to assume the responsibilities of adulthood when they are biologically ready will reduce the length of the "teen" years and encourage the formation of responsible adults at a younger age. Ironically, this measure can itself further the cause of social stability which moralistic social critics seek to strengthen by mere rhetoric. Certainly, an educational system which complements and responds to physical maturation must be superior to one which ignores or seeks to contradict that growth.