Ronald Haft was 18 yesterday and his favorite birthday present was his license to sell real estate in Maryland. He said he planned to pick it up before 9 a.m. and in so doing would qualify as the youngest person to be so licensed. He took this test and passed it before his birthday anniversary date but was not eligible to be licensed until he hit 18. This senior at Sidwell Friends School has a mother who sells real estate for Town & Country Properties (the firm where he'll hang his license) in upper Bethesda and a father who is head of Dart Drug. But this young man speaks for himself with a confidence and a goal-setting that belie his meager years. "I've been watching real estate prices for five years.They fascinate me. I do a lot of reading and I worked last summer in the ad department of The Post," he said. He plans to work full part-time until summer vacation and then full time until fall when he plans to enroll at the University of Pennsylvania. "And I have been promised a listing on a $200,000 house," Ronald Haft said. This young man looks like a comer in realty selling.

New home sales and resales are growing stronger in this area. For instance, sales at Fairlington Villages, the condominium rehabilitation project straddling I-95 in close-in Virginia, reported 30 sales contracts on March 20 and 51 contracts in the week ending on that day. At the point the March total was 123, more than 113 in February and 96 in cold January. In 1976, the sales total was 630 and it was 688 in 1975. Tommy Grimes, director of sales and marketing, said there's an inventory on hand despite the high selling rate. Prices now average $50,000 in a range from the $30,000s into the high $60,000s. The reason for the hot selling market, as Grimes sees it: pentup demand, change of administration, some newcomers to the area --but above all else, interest rates under 9 per cent and available mortgage money. What else is new?

At Beekman Place, where the interest rate is 8.5 per cent for purchasers with 20 per cent down, Rick Leeds reported that more prospective buyers turn up at income tax time because they become conscious of the need for deductions for property taxes and interest. Meanwhile, prices have increased and a two-bedroom dwelling, formerly pegged at $63,500, now is $70,950. Some owners of two-bedroom units are renting them for $550 a month and the tenant pays the cost of utility services.

If you want a quick reading on how activity in housing has increased in the District this year, the D.C. Department of Housing and Community Development has figures showing that 2,072 new houses will be built this year, compared with 1,184 in 1976. And the number of rehabilitated dwellings will be 1,435, compared with 832 last year.