SAN FRANCISCO (Reuters) - California' population will grow at much slower pace than state officials projected as its birth rate sags and immigration levels off, a study said on Tuesday, with significant ramifications for the most populous U.S. state's finances and economy.

The University of Southern California's Price School of Public Policy report's population projections, based on 2010 Census data, differ markedly from state projections made in 2007 by California's Department of Finance.

"The population level previously expected for 2020 is not reached until 2028 (44.1 million). And the 50-million population mark previously anticipated for January 2032 is now expected in January 2046, fully 14 years later," the report said.

At the same time, the most populous U.S. state will be growing markedly older, with population growth among its seniors ages 65 and older projected to quadruple in the coming 20 years, the report said.

"The senior ratio is taking off in California," said Dowell Myers, coauthor of report, adding that aging Californians will become increasingly dependent on the state's homegrown population instead of immigrants.

"Driven by aging of the large Baby Boom generation, the ratio of seniors ages 65 and older to prime working ages (25 to 64), is projected to soar to 36.0 seniors per 100 working age in 2030, compared to 21.6 in 2010, a two-thirds increase in just 20 years," the report said. "The impact of the increase is amplified because it follows four decades of no change in the senior ratio."

A growing senior population could alter California's finances and economy dramatically.

"As this ratio slowly rises, it will gradually tip the scales toward more emphasis on behaviors that the elderly are likely to engage in — not simply retirement but consumption of public entitlements, reduced taxpaying, and increased home selling," the report said.

"A two-thirds increase in the ratio of seniors to working age Californians seems certain to impose enormous pressure on state and local governments and the taxpayers. A lot is riding on the shoulders of the new generation of young adults," the report said.

It forecasts the share of California's population made up of children will slowly decline to 20.7 percent in 2030 from 24.9 percent in 2010.

(Reporting By Jim Christie; Editing by Dan Grebler)