THE GOOD NEWS: serious crime is down in the Washington suburbs, and crimes against persons, as distinct from property, are down dramatically in the District. Better news: these figures will probably decline further without any special government programs or spectacular new crime- fighting techniques. The reason is pure demographics. In the next few years, the population of 18-to 24-year-olds--those most likely to commit violent crime--will begin to decline.

In 1980, three-quarters of those arrested nationally for violent crimes--murder, rape, robbery and assault--were under 25 years old. Another 16 percent were between 25 and 29. These are the dangerous years. Young adults are not yet settled in their personal lives and unemployment is high. Some turn to crime--the kind of violent crime the public fears most.

After 30, the crime rate falls off sharply. Some of this is due to the fact that the worst offenders are already in prison. Others have settled into jobs and law-abiding lives. Your average mugger is not 47 years old. He's 18 to 25.

Look at the census figures. Today, there are about 6 million more Americans between 18 and 24 than there were a decade ago. But there are 61/2 million fewer children under 13 than there were at the time of the 1970 census. As these children move into the dangerous late teens and early 20s, crime statistics should fall off simply because there will be fewer potential criminals.

In the Washington suburbs, police offer further explanations for the lower crime rate in the first quarter of the year. The severe cold was a factor. Increased home security measures and good police work played a part. These should be encouraged, for whatever the demographic data, crime will always be a source of great anxiety. It is comforting, though, to look at the population projections. If current crime patterns are any guide, there is reason to hope we will see a significant decrease in violent crime before the next census.