FRESNO, Calif. — Arthritis doesn't stop 89-year-old Shigeo Yokota from climbing on a tractor and tilling the orchards and vineyards around his Kingsburg, Calif., house.

Nor does a thermometer reading above 100 degrees on his shaded porch keep him from walking his farm to open and close irrigation valves, giving his trees and vines a welcome drink.

But time eventually will catch up to Yokota, as it has his graying profession.

In California — and across the United States — growers like Yokota, on farms large and small, are getting older.

A fourth of American farmers are 65 or older. Half are 55 or older.

The average age of California farmers moved from 53.2 years old in 1974 to 56.8 in 2002, the last year the federal government conducted an agricultural census.

"They're unable to retire," said University of California Extension economist Steven Blank. "They can't afford it. If the sole source of income is the agricultural operation, and they have no retirement or pension, they reinvest profits to expand or update facilities."

Otherwise, he said, they may sell off assets — such as land. And that, of course, is the source for future farming income.

The trend has significant effects:

Farmers sticking around longer sometimes means deferred dreams for their would-be replacements. In some cases, there's just not enough profit from the family farm to sustain partners. The younger generation may not end up as the "farmers" — owners of farming enterprises — until they're in their 40s or 50s. By then, many have already left to pursue other opportunities.

It means tough choices for family members sorting out how to keep an enterprise going as age takes its toll on the founding generation. When the time for a change in leadership comes, it can be too late to save a run-down farm.

The U.S. Department of Agriculture has recognized the issue, making several recommendations for the next Farm Bill. They include calling for $250 million in payments to beginning farmers and ranchers, reserving them a percentage of conservation funds and providing more federal loan flexibility.

Still, pressures on aging farmers mount: urban encroachment, increased regulation, rising fuel and land prices, and adverse weather that can ruin a season.

But not all is bleak. There are signs that interest in farming hasn't waned.

Today, there are 130,000 members of 4-H clubs, compared with 62,000 20 years ago.

Enrollment remains high in agricultural disciplines at universities.

Many in farming's "next" generation may be keeping their hands in the industry if not the soil.

"Some may be in allied industries — seed sales and banking, for example — waiting for the time when they can get back into production ag," said Sarah Mora, program director for the Young Farmers and Ranchers Program with the California Farm Bureau Federation.

She uses herself as an example. A fourth-generation member of a ranching family in Humboldt County, Calif., the 34-year-old has had to make her living outside the ranch because older family members remain in the business.

Like Mora, many remain on the sidelines, waiting because of other obstacles that extend beyond finding a self-sustaining niche in the family business.

Glenn Yokota, 52, does most of the farming and management on both properties as his father has reduced the length of his workdays. But the younger Yokota also works full time at the University of California Kearney Agricultural Center as a staff research associate with the University of California at Berkeley.

"My strategy now is to break even on the farm," Glenn Yokota said, adding he has no plans to retire from his day job anytime soon.

Shigeo Yokota said there are those enterprises — including large farms and packinghouses — where multiple family members can thrive.

The elder Yokotas say their longevity is due in part to staying active on the farm they treasure.

And surviving challenges.

During World War II, Rosie spent three years in a relocation camp in Gila River, Ariz.

Shigeo enlisted in the Army before the attack on Pearl Harbor, worked in counterintelligence in the South Pacific and narrowly missed boarding a military transport plane bound for Okinawa that crashed, killing all aboard.