It was a milestone: The census numbers released last week show that one out of every four San Diego County residents is Latino.

A decade ago, it was one out of five. And the population is increasing fast.

So who are Latinos?

They're a diverse group. They're united by language, culture and history but can be of any race or any of the various Latin American nationalities. Their families have been here for generations or have just arrived.

And, the census shows, they are more likely to be younger and of child-bearing age than other population groups.

In San Diego County, they're Mexican immigrants like Domingo Cruz, 22, a gardener who lives in Escondido, or Dulce Alvarez, 18, an honor student at Bonita Vista High School in Chula Vista or her father, Miguel, 42, who teaches optical electronics at the University of California San Diego.

They're Mexican-Americans like Laura Berlanga, 22, an international business major at San Diego State University, or Chicanas like Sylvia Romo-Lara, 31, a nurse in training from Los Angeles and the doting mother of two children, aged 4 and 8, to whom she and her husband gave ancient Aztec names.

They all migrated to San Diego County in the 1990s and are part of the reason the region's Latino population rose nearly 50 percent in the past ten years.

The county's Latino population boomed from 510,781 to 750,965, continuing a decades-long trend fueled by steady immigration and high birth rates, according to the census. Nationwide, the Latino population grew 60 percent to 35.5 million and may be large enough to equal or surpass blacks as the nation's largest minority group.

One in three of the nation's Latinos live in California.

In San Diego County, Latinos have the highest concentration in National City, where they make up 59 percent. In Chula Vista, they are almost 50 percent; Imperial Beach, 40 percent; San Marcos, 37 percent; Vista, 39 percent; Escondido, 39 percent; Oceanside, 30 percent; and San Diego, 25 percent. These cities showed substantial growth in the 1990s.

The census also revealed that Latinos are more segregated from whites than they were in 1990, according to a statistical measure of segregation called the "dissimilarity index."

In 2000, San Diego County had a "dissimilarity index of 50, meaning that 50 percent of Latinos would have to move to new neighborhoods to make the county fully integrated. In 1990, the county's index was 45.

Immigration added to the region's Latino population but didn't account for more than "20 percent of the increase," said Fernando Soriano, director of the National Latino Research Center at California State University San Marcos.

"The majority was due to births, and that's just natural growth."

Or as Romo-Lara likes to explain it: "It's family. We love family."

Indeed, Latinos, who make up one third of California's population, have accounted for nearly half of the state's births in recent years.

The Latino profile that emerged from the federal census last week showed a population that is getting younger.

More than 36 percent of San Diego County's Latino population is under 18; it's 21 percent for non-Latinos. The Latino population is expected to continue to grow as a result.

Latinos are projected to make up a third of San Diego County's population by 2020, according to the San Diego Association of Governments.

Nowhere is the growth in the young Latino population more evident than in the county's public schools, where the number of Latinos grew from 100,480 to 184,792 during the past decade.

Latinos make up 37.8 percent of the county's public school enrollment and are expected to become the largest group within this decade, the County Office of Education said. Statewide, Latinos make up 43.2 percent of California's student population.

While San Diego County has become a fountain of youth for Latinos, the flow of progress for this population has been severely slowed by poverty and other economic issues.

Some factors to consider about the county's Latino population:

 The number of low-income students qualifying for free or reduced price lunches in public schools doubled in the 1990s, as did the number of limited English speakers rose to more than 100,000. Most of these students are Latino.

 Latino graduation rates lag behind those of other groups. Only 58 percent of Latinos graduate high school in the county, compared to the average of 70 percent, according to the California Department of Education.

Nationwide, 11 percent of Latinos 25 years or older are college graduates, compared to 26 percent for whites, according to the U.S. Census Bureau.

 There is a rising Latino middle class in California but it has been overshadowed by the bulk of Latinos, mainly immigrants, struggling to make ends meet. In 1999, the state's median income for Latino households was $31,200, nearly half of the total for white and Asian households, according to the state's Department of Finance.

Researchers and policy-makers said the census spotlight on the young Latino population has focused attention on California's future work force. Latinos made up more than 25 percent of the county's labor pool, according to the 1990 census.

"More than a third of the Latino population (nationwide) is under 18," said Sonia Perez, a researcher at the National Council of La Raza. "That's extremely significant. These kids are going to be the future taxpayers. They're going to be the future work force."

San Diego County Latinos are increasingly becoming integrated in the region's political, business and civic life. The county ranks among the top 10 places in America for Latinos to start businesses, according to the San Diego County Hispanic Chamber of Commerce. Latinos are also voting in greater numbers and Latino candidates are winning elections to public office.

Yet many remain trapped in low-waged service industry jobs, like 28-year-old Yesenia Chavez, who marched with fellow janitors Friday to protest low wages. "It's just not right," said the Spring Valley resident, who earns $6.40 an hour cleaning offices in La Jolla.

Domingo Cruz also earns low wages, but he has no plans to return to his native Mexican state of Michoacan because he feels at home here after only a few years.

"At first it was difficult here but now it's a little easier," he said, citing the vibrant Mexican culture in North County as one of the reasons. The slender young man is a member of the dancing group Ballet Folklorico de Escondido, which performs at events around the county.

About two-thirds of the nation's Latinos are of Mexican descent. The 1990 census found that more than 85 percent of San Diego County's Latino population is Mexican or Mexican-American.

Laura Berlanga was attracted to the region because of its large Mexican presence and San Diego's proximity to the border. "It's a good place for imports and exports," said the international business major.

Dulce Alvarez and her brother Abraham, 13, had no choice but they're making the most of their opportunities. They were growing up in Mexico when their parents decided to move to San Diego in the early 1990s.

Dulce was recently accepted to the University of California Los Angeles and Abraham wants to be a doctor. Their parents call their move a wise decision.

"There are many opportunities to work," he said.

And, he might add, they are not alone.

Staff writer David Washburn contributed to this report.