So you're between 25 and 35 years old and you don't drive a BMW, sip $15 bottles of Chardonnay, send your only child to a posh private school, or "access" your broker on a home computer?

Most of the Baby Boom generation -- those persons between about 20 and 40 years old -- don't do those things either, according to a study commissioned by the Joint Economic Committee.

The study, prepared by a University of Maryland professor and an official of The Urban Institute, said that young members of the middle class have experienced a dramatic decline in their ability to achieve a home, financial security and education for their children, and that male wage-earners had slower income growth than their fathers.

"In the decades prior to the 1970s, children expected early on to live better than their parents," the report said. "Such is not now the case."

The young middle class has been able to acquire some trappings of the good life, however, because more families have two spouses working and young families have fewer children to consume income growth. They also buy smaller homes than their parents did and receive money from relatives for buying them, the study said.

Looking at housing and other economic variables, a picture of deteriorating economic standards emerges, the report said. "This picture contains relatively few yuppies -- those famous young couples who buy expensive imported automobiles and regularly eat at up-scale restaurants," the report said. "Yuppies are not a fiction. The Baby Boom generation is very large, and even the small percent who are well-to-do comprise a large enough number to make a strong impression on journalists, car dealers and restaurateurs. But while yuppies are not fiction, neither are they typical."

Last year, the typical young family headed by a person between the ages of 25 and 34 consisted of a husband and wife and one child under age 12. Fewer than half owned their own homes. The median pre-tax family income was $25,157, "hardly enough to buy a BMW and eat out regularly," the report said.

According to the report, a man at age 30 in 1949 had average yearly earnings (in 1984 dollars) of $11,924, which increased 63 percent by the time he was 40. A man age 30 in 1959 had an income of $17,188, which rose 49 percent 10 years later.

A 30-year-old in 1973 earned $23,580, which declined to $23,395 by age 40. By 1984, the earnings of a 30-year-old man had dropped to $17,520.

Since 1973, costs of many necessities have increased, the report said. For example, a 30-year-old man paid 21 percent of his monthly gross pay for a median-priced home in 1973 compared with 44 percent last year.

Since 1973, young families pay 54 percent more for home fuel and utility costs, and 65 percent more for gasoline and motor oil -- a total of nearly $1,000 a year more.

Young families in 1981 spent less on furniture, clothes, personal care and charitable contributions than a similar family in 1973 and, "contrary to popular belief," the average young family in 1981 spent only $47 more each year on eating out, the report said.

Young families in the 1980s saved less than their counterparts in the 1970s and their debt has increased, while consumer loan interest rates are 50 percent higher than a decade ago, the report said.

To buy homes now, young families get help from their parents, the report said. "In 1980, almost one-third of all first-time home buyers got financial assistance from their relatives compared to less than 10 percent in 1978," it said.

But the recent economic recession "also affected families' abilities to transfer money to their Baby Boom relatives," suggesting that the peak of that trend may have passed, the report said.

Young families also are buying smaller homes than similar families 10 years ago. "In the 1979 to 1983 period, the percent of new units with garages and with two or more bathrooms declined, as did the median number of square feet per new unit," the report said. In a series, half of the numbers are larger than the median and half are smaller.

Homeownership rates for households headed by persons under the age of 25 dropped from 23.4 percent in 1973 to 19.4 perent in 1983, the report said. For households headed by persons between the ages of 25 and 29, homeownership in that period declined from 43.6 percent to 40.7 percent.