Builders expect a growing number of their customers in the next five years to be over 65 or homeowners buying newer, larger houses, according to a survey of delegates attending the National Association of Home Builders' annual convention here.

Young, first-time buyers, who once bought most of the houses constructed by many builders, are being crowded out of the market by higher costs and tighter standards for getting mortgages, NAHB members said.

In a forecast of the future for the housing industry, the NAHB predicted that "trade-up" buyers will constitute 65 percent of the new housing market in the next 15 years as "the baby-boom generation moves up the economic ladder. . . and their income and family size increases," according to the forecast.

Homeownership rates nationally have dropped by nearly 2 percent since 1980, primarily because there are fewer homeowners under 35, the NAHB said. Ownership rates overall are likely to rise from the current 63.9 percent of the population during the rest of the decade, "but young household trends are likely to stay where they are or get worse," according to the association's forecast.

Many young Americans, however, still expect to buy their first homes in the next few years. When they do, they will look for "a contemporary version of the same house their parents bought 25 years ago -- a single-family home with three bedrooms, two baths and a yard" and located in the suburbs, according to a survey commissioned by Carole Eichen Interiors Inc., a California-based design and marketing firm, that was described at the convention.

Two-thirds of the builders taking part in the convention survey said they expect to construct more houses this year than they did in 1985 and to charge more for them. The delegates were more optimistic than the NAHB's leaders, who issued thur Bartow 70s and will live another 12 to 15 years, according to Barbara Kleger, director of development for Lifecare Communities Corp. in Bala Cynwyd, Pa. Two-thirds of them are single or widowed, and two-thirds of these are women, she said.

She said that salespeople, when trying to convince would-be customers to buy a unit in a community for the elderly, should remember that these persons "have recognized and admitted they are growing older, and that's not easy for people to do. They realize this is the last move they will make and that's scary to them."

Although building for the elderly is the fastest growing segment in the industry, second-time buyers are the largest group.

About 60 percent of the builders surveyed said they are targeting their homes to "move-up" buyers now, putting them in agreement with the findings of a study conducted by the NAHB economics division.

The convention survey showed that 63 percent of the builders questioned expect to target their homes for the trade-up market by the end of the decade.

David C. Smith of McDowell, Va., the incoming president of the NAHB, said the Dallas convention was "the most successful" in the association's history, with a record attendance of more than 55,000 persons. A major NAHB goal in 1986 will be "to identify the depressed housing markets" around the country "and bring them back to life," Smith said. The association also will place more emphasis on research and developing new technology for the industry, he said.