Declining migration and falling birthrates have led to a drop in the number of children in California just as baby boomers reach retirement, creating an economic and demographic challenge for the nation's most populous state.

"After decades of burgeoning population and economic growth…the state now faces a very different prospect," said a report released Tuesday by the University of Southern California and the Lucile Packard Foundation. The report, "California's Diminishing Resource: Children," analyzed data from the 2010 census and the American Community Survey to conclude that the trend marks a "historic transition" for the state.

In 1970, six years after the end of the baby boom, children made up more than one-third of California's population. By 2030, they will account for just one-fifth, according to projections by lead author Dowell Myers, a USC demographer. "We have a massive replacement problem statewide," Mr. Myers said in an interview.

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