With baby boomers preparing to retire as the best educated and most skilled workforce in U.S. history, a growing chorus of demographers and labor experts is raising concerns that workers in California and the nation lack the critical skills needed to replace them.

In particular, experts say, the immigrant workers needed to fill many of the boomer jobs lack the English-language skills and basic educational levels to do so. Many immigrants are ill-equipped to fill California's fastest-growing positions, including computer software engineers, registered nurses and customer service representatives, a new study by the Washington-based Migration Policy Institute found.

Immigrants -- legal and illegal -- already constitute almost half of the workers in Los Angeles County and are expected to account for nearly all of the growth in the nation's working-age population by 2025 because native-born Americans are having fewer children. But the study, based largely on U.S. Census data, noted that 60% of the county's immigrant workers struggle with English and one-third lack high school diplomas.

The looming mismatch in the skills employers need and those workers offer could jeopardize the future economic vitality of California and the nation, experts say. Los Angeles County, the largest immigrant metropolis with about 3.5 million foreign-born residents, is at the forefront of this demographic trend.

"The question is, are we going to be a 21st century city with shared prosperity, or a Third World city with an elite group on top and the majority at poverty or near poverty wages?" asked Ernesto Cortes Jr., Southwest regional director of the Industrial Areas Foundation, a leadership development organization. "Right now we're headed toward becoming a Third World city. But we can change that."

How to respond to the inexorable demographic trends is a question sparking a flurry of studies, conferences and new programs. This week, a USC conference featuring Cortes, former federal housing secretary Henry Cisneros and other community leaders will explore ways to help immigrants better integrate into career-oriented jobs and civic life.

The Los Angeles Community College District has launched a workforce development committee of city officials and community leaders to figure out how to better prepare students for skills needed in the region.

Last week, more than 500 people gathered at Crenshaw High School at a conference sponsored by One LA-IAF, a network of more than 100 churches, unions and community groups. The network has launched a collaboration with community colleges and employers to recruit low-wage workers, many of them immigrants, and train them for jobs as nursing assistants and solar-panel installers.

"Our vision is to create a seamless program that takes undereducated, underemployed and underskilled workers and puts them into education and job training that will connect them to career ladders that pay well and offer benefits," said Yvonne Mariajimenez, a One LA leader. "It's really rebuilding the middle class."

One of the current trainees is Wendy Estrada, a 30-year-old Honduras native and naturalized U.S. citizen who aspires to move from her current work as a house cleaner to a certified nursing assistant and ultimately to work as a licensed vocational nurse.

Estrada first learned about the program at her parish, St. Thomas the Apostle Catholic Church in Los Angeles. One LA organizers came to recruit members for the pilot program, funded by the state community college system, offering free classes to upgrade English and math skills to levels required for a certified nursing assistant course.

Estrada used to dream of becoming a doctor in Honduras before marriage, motherhood and work struggles in Los Angeles waylaid those plans. She learned English soon after legally arriving in Los Angeles to join her mother in 1998, so determined to master the language that she went to classes morning and night, five hours a day, for a year.

When the recruiters came to her parish, Estrada immediately applied.

"I loved the slogan, 'Building a bridge to a new and brighter future,' " she said. "I knew that education can improve your life, but it was like I fell asleep having to work, pay the rent and just survive. This program has reawakened my dreams."

For four months, she received training at Los Angeles Valley College in basic math and English skills geared toward healthcare work -- calculating a baby's head measurement, for instance -- along with skills in time management, meeting goals, interviewing and job hunting.

In February, Estrada began her certified nursing assistant course, where she has gained both practical skills and academic knowledge. On a recent afternoon, she huddled over another student posing as a patient while trying to figure out which way to turn him to remove a protective bed pad. Instructor Dory Higgins strode over, took a quick glance and showed Estrada the right technique.

"You change the soiled pad immediately, before you do anything else, because of skin damage," Higgins said as students took notes.

The growing import of immigrant workers is reflected in Higgins' class. Among 21 students in class that day, 17 were immigrants from a range of countries: Mexico, India, Guatemala, Egypt, Honduras, Indonesia, Cuba, Congo, Ukraine and the tiny West African nation of Burkina Faso.

One of them was Maria Reyes, 30, a Guatemala native who signed up for the program through her church. Reyes, who came to Los Angeles at age 5, graduated from high school and got pregnant at 18; she began working as a waitress to bring in money for her new family. Her minimum-wage job, however, was leading nowhere.

"I wanted to keep doing something better," Reyes said. "This way I'm helping my family and other people with needs too."