The Washington Post incorrectly reported yesterday that the number of crimes committed by juveniles in Montgomery County rose 40 percent in the first half of 1979 over that of last year. County police say they have no precise statistics on the number of crimes committed by juveniles.

Crime is increasing at a faster rate in Montgomery County than in the District of Columbia -- a surprising trend that experts attribute in large part to a surge in crimes committed by juveniles in the wealthy suburb.

For the first six months of this year, according to FBI statistics, Montgomery County's crime rate was 19.7 percent higher than it was a year earlier, compared with a surge of only 15.6 percent in the District. Juvenile crimes in Montgomery over that same period rose almost 40 percent.

"You've got to understand that the suburbs, which were founded in the '50s, now have a lot of teen-age kids, and they're the ones committing most of these crimes," said Stanford University criminology professor John Kaplan.

Indeed, the FBI statistics show a marked increase in crimes committed by suburban youths in recent years, but only a modest increase in the crime rate of juveniles who live in Washington and other urban areas. There has been a steady increase in the number of rapes, robberies, homicides and car thefts committed by teen-agers in the suburbs since 1976. In urban areas, however, only the arrest rate for youths involved in car thefts has increased.

FBI spokesman Dave Cassens said arrests of youths under 18 for crimes of "commercialized vice and prostitution" in the suburbs jumped 95 percent from 1976 to 1977.

In Montgomery County, which Cassens described as "one of the wealthiest areas of the world," the increase is most profound in property crimes and "crimes of opportunity," such as burglary, larceny and car thefts. Bike thefts rose 36 percent, shoplifting about 20 percent and car thefts about 28 percent the first half of this year.

"We don't have anything serious as far as organized car theft rings go," said county detective James Eckenrode. "Most of our problems have to do with so-called joy-riding. You know, someone takes a car for a little spin and then leaves it somewhere."

The lawbreakers, Eckenrode said, are "mostly young male kids, mostly from within the county.Whether it's because the kids are bored, or what, I don't know."

The FBI's 1978 Uniform Crime Reports said the typical person arrested for car theft was male and 15 to 19 years old. Sixty-four percent of those charged with car theft in 1977 were referred to juvenile courts, a higher percentage than for any other crime.

Juveniles are also to blame for another Montgomery County crime problem, vandalism, which rose 8 percent the first six months this year.

"It seems like vandalism's always increasing," said Lt. Joseph Hancock of the county crime prevention unit. "And it continues to be a kid problem."

Rose Lederer, a resident of Friendship Heights near Chevy Chase, said vandalism is the only type of crime she has noticed in her area.

"For awhile, someone was putting soap in the fountain in the park outside our window," she said. "Then -- I don't know how they did it -- but they bent the flagpoles and uprooted a concrete bench. It was a pattern for a few weeks in a row, usually happening after midnight on Friday nights."

Nationwide, suburban crime rose by 13 percent for the first quarter, a significant increase after a decrease two years ago. In comparison, the overall national crime rate for the first quarter was only 11 percent.

National statistics for the first six months of this year were not available.

Montgomery County police tend to attribute the local increase to a rise in the number of crimes reported rather than in the number actually committed. They see no specific crime trends and have not implemented any special program to deal with the increase.

"We have different task forces operating, but nothing special," said Lt. Paul Hrapchak, head of Montgomery's Crimes Against Property unit.

Experts say that while the crime rate has risen more shaprly in the suburbs, more crimes are still being committed in the cities.

Cassens explained: "Statistically, the increase is so high in part because you have a lower percentage base, less people. So if you have two murders in '77 and four in '78, you get an increase of 100 percent."

"But it's still scary as hell," he said.