For the third successive year, most crimes decreased in Prince George's County. Figures for 1984 released by county police show an overall drop of 5.3 percent. This follows a national downward trend in reported serious crime.

Prince George's Police Chief Michael J. Flaherty yesterday attributed the downward spiral to the fact that "there are less people in the crime-prone ages [of 15 to 24] in the country" because of a birth rate decline that began 20 years ago.

Juveniles and young adults are by far the largest group of offenders, Flaherty said.

The chief also said there has been a national push in the last 10 years by police departments to enlist citizens in the fight against crime.

Flaherty credits programs like Operation Identification, Neighborhood Crime Watch, and Crime Solvers with helping curb crime, because, he said, people are now more aware of the potential for crime and consequently are more careful.

"Not to mention," Flaherty added, "we have extremely good law enforcement agencies in the Washington area."

Police departments in this area, he said, "tend to try new ideas earlier." He cited drunk-driving road checks initiated by Montgomery County police.

According to the newly released police statistics, reported crime in the county dropped in all categories except rape, aggravated assault and car theft.

There were 39 murders in the county in 1984, compared to 55 the previous year. Robberies dropped about 15 percent during the same period.

But rapes increased by 13.5 percent, aggravated assault by 10.9 percent and auto theft by 4.6 percent.

According to the FBI, serious crimes decreased statewide in Maryland as well as in the nation in 1983, and the agency expects that trend to repeat itself when the final 1984 figures are compiled.

Bernard D. Crooke, Montgomery County police chief, said that the 4 percent increase in crime last year in his county was no cause for panic.

In looking at statistics covering the 10 years from 1974 to 1984, there is still a general decline in crime, Crooke said.

Crooke also said he is particularly worried about the large number of juveniles being charged with serious crime. In his county last year, he said, 30 percent of those arrested were juveniles.

Jail officials in Prince George's also report an increase in youngsters who have been charged with serious crimes.