A recent Wall Street Journal article highlighted the challenges that will be facing California, due to

Declining migration and falling birthrates [that] 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.

California hasn’t always faced demographic challenge. The article continues:

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.

Demographic decline is a subject the Marriage and Religion Research Institute has analyzed in detail. In his work on the decline of economic growth and population change, Dr. Henry Potrykus looks at the slowdown of GDP growth due to declining numbers of high-human capital wage earners, and he predicts that the U.S. economy will continue to see growth ebb over the coming years. “This slowdown,” Dr. Potrykus says, “is amplified by the retiring of a generation with significant human capital (the baby boom) and its replacement by a generation inadequate in population size to continue the expected and required growth of the macroeconomy.” In other words, the U.S. population will not be able to replace its retirees with an equivalent number of skilled adult workers, due in part to low birth rates.

As the Wall Street Journal Article notes,

[California’s] birthrate fell to 1.94 children per woman in 2010, below the replacement level of 2.1 children, according to the study.California's rate is lower than the overallU.S.rate of 2.06 children in 2012, according to the Central Intelligence Agency.

“This population trend is a significant problem nationally when close to two million people will retire each year for the next 20 years, according to Stephen Levy, director of the Center for Continuing Study of the California Economy.”

As goes California, so goes the nation.