Serious crime was down in the District of Columbia and the suburbs for the first three months of this year compared with last year, according to area police, following a nationwide trend.

Police in the metropolitan area cited tougher sentencing of convicted criminals and beefed-up police forces as reasons for the marked decrease in crime here.

Criminologists credited the aging of the baby-boom generation, and the fact that there are proportionately fewer persons aged 15-29 in the population, the ages they say are most likely to commit crime.

Neighborhood watch programs, which have become increasingly popular in the area, have also helped lower the number of serious crimes, police said.

Montgomery County reported a 16 percent overall decrease in serious offenses--murder, rape, robbery, burglary, larceny, aggravated assault and car theft. Prince George's County registered a 13 percent overall decline, the District 11 percent, Fairfax 10, Alexandria 7 and Arlington 3.

Some individual categories of crimes were up in some of the jurisdictions, but the overall pattern resembled the downturn described last week by U.S. Attorney General William French Smith.

Dr. Charles Wellford, director of the University of Maryland Institute of Criminal Justice and Criminology, yesterday attributed the decline to demographic changes.

"We are shifting from a very young to an older population," he said. "In particular, 15- to 29-year-olds have very high rates of crime. As fewer people are in those categories, there will be lower rates of crime."

Wellford predicted the downward trend will level off "somewhere near the end of the decade."

Montgomery County registered 5,217 serious offenses in the period from January through March, the lowest total since 1978. Prince George's had only one-fourth the number of burglaries it experienced a year earlier, Arlington one-third the number. The District cut the number of robberies by 10 percent and reduced burglaries by 20 percent.

Montgomery County Police Chief Bernard Crooke said "a no-nonsense approach to sentencing by the courts, a concerted effort by the state's attorney's office to prosecute felonies and police targeting of high-crime areas" were some of the reasons crime is down.

Prince George's Police Chief John E. McHale cited his department's addition of 100 officers since July 1981 as the major reason for the decrease in crimes there.

That and the neighborhood crime watches, he said, "have had a direct bearing on the decline in house breakings," which are down 30 percent for the quarter. In Prince George's only aggravated assaults were up, by 13 percent, and McHale speculated that the recession may be the cause.

The number of murders dropped in Prince George's from 19 in the first quarter of last year to eight for the same period this year, in Montgomery from six to one, and in Arlington from three to one.

Murders increased in Alexandria during that same time from none to 4, in Fairfax from one to four and in the District from 58 to 59.

The number of reported rapes dropped in all area jurisdictions except Arlington and Fairfax. The largest decline was in Montgomery, where 40 rapes were reported in the first quarter last year and 19 in the same period this year.

There were 44 fewer armed robberies in Montgomery, a drop of 34.4 percent. Residential burglaries there were down by 261 offenses or 26 percent and commercial burglaries were reduced by 65 or 15.1 percent, according to police spokesman Cpl. Phillip Caswell.