It was just past 9 p.m. on a Friday night, so Pat Brown, like any self-respecting 24-year-old party boy, made a beeline for the hottest bar in town. But when he arrived at the Tavern, considered the trendiest nightspot in central Calvert County, barely a handful of the 75 or so patrons were within a decade of his age.

A 36-year-old mother of two warbled a karaoke song about diapers and the PTA. A 51-year-old electrician with thinning hair shot pool in the corner. And two happily married women with bangs and sensible shoes sipped beer at the bar.

"It's like I'm living on a different planet," said Brown, who grew up in the area. "Sometimes I feel like there's no one else my age in this whole county."

Where have all the twenty-somethings gone? Even as this Southern Maryland peninsula became the fastest-growing county in the state during the 1990s, the number of people ages 20 to 29 dropped by 746. Only 8.8 percent of Calvert residents are in their twenties, according to the 2000 Census -- down from 15.3 percent in 1980, a more precipitous drop than the slight decrease nationwide in the percentage of people in that age group. The national average is 13.6 percent.

The generational shift in Calvert -- which along with Fauquier County had the lowest percentage of adults in their twenties in the Washington region in the 2000 Census -- is in part an extreme example of a phenomenon found throughout the region's outer suburbs: Young people are fleeing.

It's nothing new that bright-eyed high school graduates quit their rural communities to make their marks in a big city. But what is different today is that many exurban twenty-somethings who want to remain in their home towns can no longer afford to, affordable housing advocates say.

"We're forcing our next generation out of the area," said Linda Bracey, whose 23-year-old daughter lives in Bracey's Huntingtown house because she can't afford to buy or rent a home. "Pretty soon, half of us are going to be under 18, and the rest of us are going to be over 50."

In exurbs such as Calvert, nobody used to talk much about affordable housing. When Bracey moved to the county in 1977, it was filled with vast expanses of undeveloped land and scores of cheap homes. She and many of her neighbors supported slow-growth policies to prevent outsiders from inundating the county and turning it into a metropolis.

Now, the children of those exurban pioneers are in their twenties and unable to find homes. Some are moving to closer-in Washington suburbs where they can rent an apartment -- apartment rentals in Calvert are almost nonexistent. Others have moved out of the region altogether, buying houses in such places as West Virginia and North Carolina.

An advocacy group, Housing for All Calvert, was formed this summer to push the county to address the problem.

"What we're experiencing now with the cry for affordable housing is the panic from this second generation," said Olivia Campbell, 24, who lives with her mother in Dunkirk. "Now those kids are like, 'What are we supposed to do?' "

Will Kreamer, 23, is struggling to answer that question. His family moved to Calvert when he was 3 years old and settled into a quaint $62,000 Cape Cod in the waterfront community of White Sands.

"Nobody wanted to live here back then. It was a no man's land," said his father, Bill Kreamer. "Anybody could get a good deal on a home."

Today, their house is worth about $280,000 -- which seems like a bargain compared with the reported $349,375 average home price in Calvert.

Will Kreamer said he can't afford a house in the county, and he has thought about renting an apartment in Northern Virginia. But he can't bring himself to leave the community where he grew up.

"I love this county. It's beautiful. It's on the water. It's not real crowded," said Kreamer, who works four days a week at his family's produce stand and is studying for a master's degree in public policy at George Mason University. "But there's a serious lack of affordable housing, and it's getting worse."

So to make ends meet, Kreamer is living in the tiny bedroom where he grew up. On a recent weeknight, as his father stirred pungent chicken soup in the kitchen, Kreamer glanced at a poster of an underwear-clad Homer Simpson hanging on the wall of his room. A homemade African drum rested in the corner. Phish CDs spilled across his bureau.

He sighed. He doesn't want to live with his parents anymore. "I'll probably have to move out of Calvert next year to find my own place," Kreamer said. "Where? I have absolutely no idea."

The number of Americans in their twenties decreased by 21/2 percentage points during the 1990s, a drop attributed to lower birth rates during the 1970s. But in the Washington region, the decline was felt more acutely in parts of the exurbs than in the closer-in suburbs. The number of twenty-somethings in Calvert dropped 5.4 percentage points from 1990 to 2000; in Fauquier, it fell 5.9 percentage points. In both Arlington and the District, for example, that figure decreased 1.9 percentage points in the same period.

More recent estimates from the Census Bureau suggest that the percentage of adults in their twenties in Calvert has risen since 2000, although demographers caution that those rough figures are sometimes inaccurate. Josh Schneider, 27, like other young adults in the county interviewed for this article, said he hasn't seen an increase.

The few twenty-somethings who stay in outer counties such as Calvert say life is often subterranean and lonely.

"Most of the young people I know here live in a basement," said Schneider, a high school physics teacher who rents the basement of a house owned by a married couple in Owings. Almost everyone he knows in the county is married and much older.

Although youth-oriented clubs and bars are beginning to pop up in at least one exurb -- Loudoun County -- Schneider, who moved to Calvert to teach, said night life in the county is geared toward a much older crowd. "You're not going to see musical groups popular with young people down here. Nothing like Oasis or Green Day," he said. "We only get old bands like Crosby, Stills and Nash and Bob Dylan."

"If you want to have a social life," Schneider said, "you go up to Annapolis."

Some Calvert natives such as Pat Brown try to find a semblance of a social life inside the county. On a recent Friday, he headed to the center of St. Leonard determined to find some fellow twenty-somethings and, hopefully, a good time.

A dozen of his buddies sat in a corner of the Tavern, but Brown, a musician who lives with his grandmother, said things just aren't the same since most of his friends escaped Calvert.

Many left in search of affordable housing or higher-paying jobs, he said, but a lot just wanted to move somewhere filled with people their age.

As Brown gulped his Long Island iced tea, Danielle Wilt, the 36-year-old married mother of two, got up and crooned karaoke-style to the song "Mr. Mom" by Lonestar:

Pampers melt in a Maytag dryer

Crayons go up one drawer higher

Rewind "Barney" for the fifteenth time

Breakfast six, naps at nine

Brown, who was waiting to sing a song by Green Day, took a long swig of his drink.

"Man, I've got to get out of this county," he said. "If I don't hang out with some people my own age, I'm going to go crazy."

Pat Brown sings karaoke at the Tavern, a hot spot in St. Leonard, where few twenty-somethings are found.Will Kreamer works for the family business and lives at home as he studies for his master's degree in public policy at George Mason University.