Columbia and Reston, among the few successful, privately developed new towns in America, are now teen-aged cities at the mid-point of development.

Reston and Columbia have come a long way and handled a lot of problems. Both are scheduled to be completed in the next decade, Reston with about 55,000 residents and Columbia with more than 100,000.

Both are moving confidently, if cautiously, into the 1980s, despite the economic stringencies that threaten all real estate development. Reston, in Northern Virginia's Fairfax County, and Columbia, in Maryland's Howard County, survived three previous economic downtowns. Both had outstanding records of residential and commercial growth in 1979.

"We have momentum going for us now with more diverse new projects under way, out-of-state transferees coming in and an unemployment rate of half the national average of 6 percent," said Charles Tuchfarber, Columbia's director of marketing.

"Our staff is already planning for the next upturn," says Francis Steinbauer, executive vice president of Reston Land Corp., a subsidiary of Mobil Oil Corp. A Reston resident for 15 years, Steinbauer said that light industrial and commercial development, "which were slow at first," increased rapidly in the past three years.

For instance, Sperry Management Systems Inc. is building the first of several large buildings on a 35-acre tract in bustling south Reston. Sperry plans to employ about 2,000 workers.

Columbia's 16,000 acres were assembled nearly 20 years ago by mortgage banker-developer James W. Rouse. Today, twice as large as Reston, it has 55,000 residents in 17,000 dwellings, a fourth of which are rentals.

At this mid-point in its development, Reston also has 28,000 on-site jobs, more than 900 businesses and 10 million square feet of office, industrial and retail space.

The biggest employer is General Electric, which has about 2,000 on its payroll. Among the other businesses there are Westinghouse, AMF-Head (sportswear), Scan Furniture and Subaru.

While Reston has several large neighborhood shopping areas, it has no mall, although Tysons Corner is not far.

Almost from the beginning, Columbia has had its Rouse-created mall. With Hecht's and Woodies as anchors, the mall is being expanded to include a 150,000-square-foot Sears and 133,000 square feet of new space for small retailers.

Both Reston and Columbia are about 25 miles from the White House. While location -- Rte. 29 and more recently on I-95 midway between Washington and Baltimore -- has always been one of Columbia's attractions, Reston has been out in Washington's left field, near the Loudoun County line.

However, the steady growth of traffic and business at nearby Dulles International Airport has helped Reston, which has long lusted after the benefits of the high-speed, four-lane, limited-access highway that connects the Beltway to the airport.

"Thanks to the Virginia legislature, we now anticipate new parallel lanes to serve nearby commuters by 1984, in addition to the limited access now provided for rush-hour traffic," Steinbauer said.

Reston's founder, Robert E. Simon, wanted to make the new town a place to live, work and play. It now has 35,000 residents, nearly a third of whom work there. That comes out to about a job per household.

Reston still has about 3,000 undeveloped acres. Development is strong in the "south lakes" area on the sunny side of the transecting Dulles highway. Further plans call for development in the far northern quadrant of Reston; one day a regional mall may be opened there.

Reston has a Sheraton hotel complex that serves Dulles visitors, business generated by light industries and the town's 29 trade association headquarters. gColumbia's Cross Keys Inn is getting a 10-story, 144-room annex with more conference and convention facilities adjacent to downtown Lake Kittamaqundi.

Reston's original downtown, which includes the high-rise Heron House and some contemporary and traditional town houses, is on Lake Anne.

Because Reston and Columbia are in different states, approximately 40 crow-fly miles apart and considerably farther by car, their competition for new residents, light industries and businesses have been less than cutthroat. Rivalry does exist, but it has been a healthy, arm's length competition in most instances. Both cities are essentially middle-class and especially attractive to Washington's younger couples as well as newcomers to the area.

Reston's Steinbauer likes to point out that Reston's wooded sites are more plentiful and thus accommodated more single-family, sprawling residences in early years. "I think we'll be higher density with smaller dwellings in the second half of the game," he said.

Aware that three-fourths of Columbia employes live outside the new city, marketer Tuchfarber points to an expanding downtown and an urban scale that sets dominant Columbia apart in rural Howard County.

Reston's marketing vice president James Cleveland said that the energy crunch benefits the inner city as well as emerging new towns, because people can work and live there.

Reston had a native industry before it was born: The Sunset Hills distillery of the Bowman family. It continues to turn out bourbon in a small, low-profile plant. And there's still one Bowman house in Reston, which originally was that family's Sunset Hills farm.

Columbia, in turn, is the principal home residence of its founder. Jim Rouse retired last year as chief operating officer of his firm but continues to live in a modest house in the new town. Columbia's oldest residential building firm is the Ryland Group, headed by James P. Ryan, who lives inside Columbia on a small farm.

The Ryland firm and Columbia Builders build the most houses in that new town. Both have several models in a newly opened Columbia village called Dorsey's Regard, named for a manor house on the site west of Rte. 29 and north of Rte. 108. The first section will include 388 houses priced from $100,000 to $150,000. Nine builders are already showing new models in the subdivision.

One of them, Brantly Homes, soon will be showing a new-Victorian, all-wood house with a wood foundation and passive solar energy features. Sponsored by the American Wood Council and Howard Research and Development Corp. (Columbia's developer), the house was designed by architect Robert Kaplan. The "idea house" is being furnished by the Hecht Co. and will be shown daily for five weeks beginning May 3.

A recent "idea house" was shown two years ago in Reston.

And finally, neither Columbia nor Reston has its own city government but there are myriad community organizations. Reston now has legislated authority to set up its own government. Both new towns have two golf courses -- one private and one public. Both also have a lot of tennis enthusiasts and courts.