THE TERRORIST MENTALITY. THE Shiite suicide bomber who drove her explosive-packed Peugeot into an Israeli Army convoy in South Lebanon last year was 16 years old. The Jordanian who tried to assassinate the vice-consul of the United Arab Emirates in Rome in 1984 was 22. Of the four hijackers of the Achille Lauro, all of whom were sentenced in Genoa, Italy, last November, the oldest was 23; the youngest, 19.

Terrorists, whatever their background or cause, have one thing in common: their age. ``Most of them are youngsters,'' says Prof. Ariel Merari, speaking of the Middle Eastern terrorists he has studied through the Project on Terrorism which he directs at Tel Aviv University.

Scholars looking at other parts of the world second his view. ``The age of terrorism is the age of youth,'' says Jerrold M. Post, a psychiatrist who studies terrorism as director of behavioral sciences at Defense Systems Inc., a consulting firm based in Washington, D.C. ``Apart from some of the leaders, you basically don't find middle-aged terrorists,'' he notes, citing research that puts the median age of terrorists at 22.5 years.

Dr. Lorenz Bollinger, a West German psychologist who conducted detailed interviews with imprisoned members of the Red Army Faction (RAF), sees elements of what he calls ``adolescent crisis'' among terrorists.

Experts remain divided about whether a particular terrorist mentality can be defined. On this one point, however, there is general agreement: The hand that actually carries the bomb or pulls a trigger is a young hand.

Why? In some cases, the answer is relatively simple. ``In certain countries,'' says Prof. Paul Wilkinson of the University of Aberdeen, ``whole families pass on to their children the message that this is the way you struggle for your rights, this is the way we work for the cause.'' The result, he says, is a ``terrorist tradition'' of inducting children into paramilitary activity at an early age.

Beyond that, students of terrorism can trace numerous threads. Two broad types of terrorism

One one hand, Dr. Post points to the ``nationalist-separatists,'' exemplified by the Spanish Basque group ETA and the Armenian Secret Army for the Liberation of Armenia. They live openly in their own communities -- they are often lionized as heroes -- and their goal is to establish a separate nation, usually as an act of loyalty to parents who have suffered under earlier political regimes.

On the other hand, says Post, are what he calls the ``anarchic-ideologues,'' members of such groups as the RAF or Italy's Red Brigades. Living underground, often in rebellion not only against the state but against their own parents, they seek the wholesale destruction of the existing social order.

Increasingly, European specialists are finding it useful to add a third category: the spillover of terrorism from the Middle East. Often referred to as ``international terrorism'' (to distinguish it from the two domestic categories), it is characterized by state support and extensive training of commandos. International terrorism involves professional terrorists like Abu Nidal, who reportedly hires out his services to various states with little concern for ideology.

Because terrorists of these three types may resort to the same means, a bombing in Madrid may look much like one in Berlin or Beirut, and the perpetrators are often lumped together under the single label of ``terrorist.'' The ensuing confusion illustrates the difficulty in finding a single definition for the term and leads to the adage that ``one man's terrorist is another man's freedom fighter.'' The result is such a welter of differing behaviors and purposes that terrorist experts are often skeptical about identifying a universal mind-set.

``I don't see any reason to assume that there is more commonality among those that resort to political violence,'' says Dr. Merari, ``than among those that resort to criminal violence.'' He adds that ``even among terrorists of the same nationality, even among terrorists of the same terrorist group, there is not much reason to assume a common personality.''

Nevertheless, he says, some common characteristics do emerge from studies of certain well-defined groups. His own research on Palestinian hostage-takers, for example, points to high levels of aggression, a disturbed family background, and early involvement in illegal activity. Religious motivation

The difficulty, and the importance, of trying to define the terrorist mentality are illustrated by one key example: the influence of religion in terrorism.

Much of the recent violence generated in the Middle East is traceable to the brand of Shiite fundamentalism of the Iranian leader, Ayatollah Ruhollah Khomeini, especially to his approval of those who die for the cause.

Is the West, then, facing a particularly menacing threat in the form of suicidal terrorists? Dr. Merari plays down the threat. ``Contrary to the image created by the media, and by the perpetrators themselves, religious fanaticism does not play a major role [in terrorism].''

Merari argues, and Israeli intelligence officials confirm, that much of the so-called suicide bombing is not carried out by fanatics who believe that by dying for a cause they will go straight to the presence of Allah. Interviews with would-be suicide terrorists who were captured alive indicate they were often tricked into believing they could escape before the bomb exploded or they were blackmailed into accepting the task, on threat of harm to their families.

Was the motive in attacking the US Marine barracks in Lebanon a religious one? In her recent book, ``Sacred Rage: The Crusade of Modern Islam,'' veteran foreign correspondent Robin Wright offers one response. In Beirut a month after the bombing, Sheikh Fadlallah, perhaps the most influential Shiite leader outside Iran (and a man often accused of blessing young fanatics about to go forth on suicide missions), told her that ``Suicide in such a way is forbidden in our religion.''

Yet the extent to which religion plays a role remains a puzzle, even among the experts. One problem: inadequate understanding of the Muslim faith among Westerners.

``Don't underestimate the religious content of Islam,'' warns Franco Ferracuti, an Italian scholar and former government antiterrorism official. ``That's the one variable that we seem to be losing sight of all the time.''

Whatever the answer, one thing appears certain: Suicide attacks represent a new and challenging threat. Lord Chalfont, a former British Cabinet minister with antiterrorist experience in Cyprus, Malaya, and Palestine, observes: ``The whole time that I have been involved in terrorist operations, which now goes back to 30 years, my enemy has always been a man who is very worried about his own skin. You can no longer count on that, because the terrorist [today] is not just prepared to get killed, he wants to get killed. Therefore, the whole planning, tactical doctrine, [and] thinking [behind antiterrorism measures] is fundamentally undermined.'' For example: The shootings last December at the Rome and Vienna airports were carried out by terrorists who had little if any thought of escaping. A terrorist mind-set?

Is there, then, a terrorist mind-set? From dozens of interviews over four months with longtime students of terrorist ideology and psychology, certain common characteristics begin to emerge. Not surprisingly, given the typical age of the terrorist, these characteristics are also often attributed to adolescence:

Oversimplification of issues. Terrorists often reduce complex issues to black-and-white. Dr. Ferracuti points to the intensely intellectualized, inward-looking, and politically naive theorizing of many groups. The terrorist, he says, ``no longer tests his ideas against reality by [engaging in] dialogue with people with different views.'' Instead, he lives out ``a fantasy war,'' convinced that he has broad support from numerous like-minded followers.

Frustration. Pent-up concerns about an individual's inability to change society figure strongly in this mind-set. Italian historian and journalist Giorgio Bocca notes: ``They have no patience, these terrorists. They are totally absorbed in their cause. One of the slogans of one of these movements [I studied] was `We want it all and we want it now.' ''

Self-righteousness. ``They believe in [their] total rectitude,'' notes Professor Wilkinson, who adds that ``intolerance, dogmaticism, authoritarianism, and a ruthless treatment of their own people who deviate from their own view are common to their mentality.''

Utopianism. Many terrorists seem to feel that a near-perfect future lies just around the corner, once the present order is destroyed. This utopianism, coupled with frustration at the slow pace of social change, often leads to political extremism on both the left and the right. ``Unless you catch this utopian aspect,'' notes Professor Ferracuti, ``it's difficult to understand the mind-set.''

Social isolation. Terrorists, says Dr. Post, are often ``people who really are lonely.'' Terrorist groups use religious or political ideals to interest potential recruits. But those taking up political violence often share traits that are less religious or political than mental and emotional.

For many of them, Post says, the terrorist group is ``the first family they have ever had.''

Assertion of their own existence. Terrorist actions are often laden with symbolic overtones, involving the choice of captives, locations, weapons, and timing. They are frequently meant to send messages -- and the message, quite often, is simply ``I exist! Pay attention to me!'' French criminologist Jacques L'eaut'e calls terrorism ``a communication system'' in which frustrated people are ``trying to express through violence their own message.''

Willingness to kill. Perhaps the most startling characteristic, however, is the coldbloodedness of some terrorist murders. Students of terrorism relate this characteristic to a harsh oversimplification that allows killers to see victims simply as objects -- a habit of mind observed among Nazi killers during the massacre of Jews in World War II. Researchers have noticed, however, that captors who hold hostages for protracted periods tend to develop a kind of bond with them that makes coldblooded murder less likely. THE INNER LANDSCAPE What makes terrorists tick? While there is no single mind-set, certain mental traits are widely observable. Oversimplification: a fanatic, black-and-white view of the world. Frustration: impatience with a society perceived to be oppressive. Selfrighteousness: dogmatic assertiveness of their own positions, intolerance of others' views. Utopianism: a feeling that a vaguely defined `perfect society' is just around the corner. Isolation: loneliness and an inability to function well in social groups. Cold-bloodedness: a willingness to murder ruthlessly, seeing victims as objects.