Almost a third of all U.S. households, including a quarter of the elderly, have money left over for luxuries after covering expenses for a "comfortable" level of everyday living, according to a study released yesterday by the Census Bureau and The Conference Board.

The elderly have traditionally been considered the poorest group in society, but the study found that 26.9 percent of all households headed by a person 65 or over have enough income to put them in the luxury class. For elderly households in the affluent group, the amount of luxury income per person is $5,633 -- higher than for any other age group.

Using 1983 Census Bureau income figures updated to 1984 dollars, the study found that of 83.9 million households, including one-person households, 31.5 percent had money to spend on luxuries after taking care of daily living expenses.

It found that of $1.7 trillion total after-tax spendable income for all households, $278 billion was left for discretionary or luxury spending after living expenses were paid. The 26.4 million households who shared that amount averaged $46,764 per household in gross income before taxes and $34,562 per household after taxes. After covering household expenses, these households had $10,525 (or $3,713 per person) left in luxury income.

For the purposes of the study, the Census Bureau and The Conference Board defined luxury income as after-tax income in excess of the amount needed to live at a "comfortable level."

Households with gross pretax income of $40,000 a year or over, making up a fifth of all households, had four-fifths of all the luxury income.

By age, the highest proportion of families with luxury income were households headed by people aged 55 to 59, reflecting the fact that people had reached or neared the top of their earnings years and in many cases no longer had children at home to support.

More surprising is the fact that over one-quarter of the aged households had luxury income, and the per capita amount for them was higher than for other age groups -- $5,633.

The high per capita level for elderly households reflects the fact that older people get special tax breaks, have no children to support and often have paid-off mortgages. Once the aged were the poorest group in society, but now, with growth of Social Security and private pension coverage, the aged are doing much better.

The study showed that the Baby-Boom generation, now aged 25 to 39, made up a third of all households but had only 27 percent of the discretionary income.

"The prevalence of yuppies is vastly exaggerated," said Fabian Linden, research director for The Conference Board, a New York-based business research group.

The study found that better-educated households were also more comfortable economically. Households headed by a person with five or more years of college were in the luxury class three-fifths of the time. By comparison, only 14.4 percent of the households in which the head hadn't finished high school were in the luxury class.

The survey found that the suburbs of major metropolitan areas have the highest proportion of families with luxury income -- 37 percent -- with their central cities having the lowest, 24.8 percent. Measuring by race, a third of all white but only 16 percent of blacks and Hispanic households had luxury income.