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.

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