Older adults are facing their own biological clock. And they can't help but wonder if they'll live to see the grandkids as their adult children put off starting families.

Alarm bells are ringing, Mary Jane Horton, 62 years old, a writer, editor and blogger who lives in Pasadena, Calif., tells the Wall Street Journal. She longs to spend time with the grandchildren she does not yet have. "We know intellectually that we have to wait, but we don't want to," she says.

It was she who likened the feeling to that of a "biological clock" for grandparents.

There's a downside to the delay. Grandparents can be a great source of child care, confidant and buddy to grandchildren. But the older the grandparent is when the child is born, the less likely that is to occur, Merril Silverstein, professor of sociology at Syracuse University and aging expert, told the Journal. It's a loss for both of them.

It's part of a growing demographic trend that has more would-be parents choosing to wait to have children. The average age at first birth rose 3.6 years between 1970 and 2006, according to the Centers for Disease Control and Prevention. Meanwhile, in 2012, a story noted that America's birth rate reached its lowest point in a quarter-century. CDC said that pregnancy rates declined 10 percent for both married and unmarried women since 1990.

Not all kids are deprived of their grandparents. One-tenth of children now live in a household with a grandparent, according to a study from the Pew Research Center. Some are being raised by those older relatives, others live with both parents and grandparents, the study said.

It noted that 7.7 million U.S. children live in households with grandparents and for about 3 million children, grandparents are the primary caregiver.

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Static Brain said that the average age today's grandparents assumed the title is 48.

They are an active group, 60 percent still working. They are young and energetic — nearly half play sports. They are also generous with the grands. As a group, they spend an estimated $52 million on the children of their children.

How that will change if they are, on average, much older as they become grandparents is a real question that time will have to answer.

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