THE push to prosecute teenagers as adults is gaining momentum in state capitals across the country.
Driven by public outcry over brutal murders and by statistics showing that violent crime among youths is rising, states are abandoning old rules that protect juveniles from treatment as adults in the legal system.
Underlying the trend, however, are some enduring questions: Is the practice curbing crime rates - and is it doing more harm than good to juveniles? Indeed, some lawyers and judges are warning against laws created in a ''lynch mob'' atmosphere.
In just the past few weeks, several cases have surfaced showing how states and courts are handling juveniles differently:
* Fifteen-year-old Joshua Jenkins faced a San Diego judge last week on charges of murdering his parents, grandparents, and 10-year-old sister in late January. Until last summer, the teenager would certainly have been prosecuted as a juvenile.
But a tough new law allowing California to send violent offenders ages 14 to 17 to adult court has changed that. In another recent San Diego case, a judge ordered a 15-year-old girl to stand trial as an adult for the stabbing death of an older woman.
* In Chicago last month, a judge sentenced a 12-year-old boy to a state juvenile penitentiary, making him the nation's youngest inmate at a high-security prison. The youth and a 13-year-old boy were were convicted of murder for dropping a five-year-old from a 14-story building after the youngster refused to steal candy for them.
* In Boston, by contrast, a Superior Court judge recently denied a motion to try a 16-year-old as an adult for the stabbing murder of his best friend's mother. The slaying has caused shock and anger in the Boston area, due to its ugliness and because the youth accused of the crime came from a ''good'' two-parent home.
''This isn't the 50s any more,'' says David Kopel, an expert on violence at the Independence Institute in Golden, Colo., referring to the hardening attitudes toward juveniles.
Last year alone, more than a dozen states passed laws to treat juvenile criminals more like adults. Many have lowered to 14 the age at which juveniles can be so tried.
Rules for juvenile court vary from state to state. But generally, unlike in an adult trial, the hearings are confidential, the number of attorneys is much greater (parents, for example, may have separate counsel), and the judge has enormous latitude to decide punishment.
Many states are also pushing longer sentences for kids and expanding the types of crimes for which juveniles can be prosecuted as adults.
Georgia, for instance, recently mandated that youths ages 14 and older charged with murder be tried as adults. California now prosecutes about three-quarters of the teens it charges with murder as adults. Last fall, Texas listed seven different capital offenses for which juveniles can be imprisoned for up to 40 years.
Mr. Kopel says there is a ''slow-moving but parallel trend in all 50 states'' to make murder a mandatory adult crime. Such a step eliminates the current practice of ''judicial discretion,'' in which a judge weighs, based on various factors, the dangerousness of the juvenile against the likelihood of his or her rehabilitation.
Many experts believe it is media coverage of sensational murders, as much as a move inside US law enforcement, that is spurring the change in the laws.
''Honestly, it is the media cases that drive the policy response,'' says Massachusetts Assistant Attorney General Jane Tewksbury.
''There is an irresistible temptation for a state legislature to do something after one of these cases,'' Kopel says.
Behind the headlines, though, is a rise in the number of heinous crimes committed by juveniles and a rise in youth violence in general. Statistics from the National Council on Crime and Delinquency in San Francisco show that in 1987 the number of violent crimes by teens per 100,000 arrests was 600. By 1993, the last complete survey, the figure was 900. Moreover, FBI statistics on crime demographics suggest that the incidence of juvenile crime could increase in coming years: Some 40 million kids - the children of the baby-boom generation - are now under age 10.
Still, a counter move exists in the legal community to curb efforts at mandatory adult trials for juveniles. Some liberals and conservatives say it is wrong, for instance, to take away judicial discretion in such cases.
''You've got hard-core 14-year-olds and 17-year-olds that would never do this again,'' says Kopel. ''Discretion is what judges are paid to exercise.''
Others believe that rather than locking up children as if they were tiny adults, secure residential treatment centers and intensive counseling would be better. Research over the past five years shows juveniles tried in adult court, where teens seem out of place, actually fare better in terms of punishment than in juvenile court, where the judge has often seen them before.
''The old juvenile-justice system is doomed,'' says Paul Martinek, editor of Massachusetts Lawyer. ''All the fine distinctions between how kids are punished are going to go by the wayside.''