“What we’ve really seen is an increase in the salt load in children’s diet,” said Dr. Bruce L. Slaughenhoupt, co-director of pediatric urology and of the pediatric kidney stone clinic at the University of Wisconsin. He and other experts mentioned not just salty chips and French fries, but also processed foods like sandwich meats; canned soups; packaged meals; and even sports drinks like Gatorade, which are so popular among schoolchildren they are now sold in child-friendly juice boxes.

Children also tend not to drink enough water. “They don’t want to go to the bathroom at school; they don’t have time, so they drink less,” said Dr. Alicia Neu, medical director of pediatric nephrology and the pediatric stone clinic at Johns Hopkins Children’s Center in Baltimore. Instead, they are likely to drink only once they’re thirsty  but that may be too little, too late, especially for children who play sports or are just active.

“Drinking more water is the most important step in the prevention of kidney stones,” Dr. Neu said.

The incidence of kidney stones in adults has also been rising, especially in women, and experts say they see more adults in their 20s and 30s with stones; in the past, it was more common in adults in their 40s and 50s.

“It’s no longer a middle-aged disease,” Dr. Nelson said. “Most of us suspect what we’re seeing in children is the spillover of the overall increase in the whole population.”

The median age of children with stones is about 10.

Many experts say the rise in obesity is contributing to kidney stones in children as well as adults. But not all stone centers are seeing overweight children, and having a healthy weight does not preclude kidney stones. “Of the school-age and adolescent kids we’ve seen, most of them appear to be reasonably fit, active kids,” Dr. Nelson said. “We’re not seeing a parade of overweight Nintendo players.”

Dr. Slaughenhoupt has seen more overweight children at his clinic. “We haven’t compared our data yet,” he said, “but my sense is that children with stones are bigger, and some of them are morbidly obese.”

Dr. Pope, in Nashville, agreed. His hospital lies in the so-called stone belt, a swath of Southern states with a higher incidence of kidney stones, and he said doctors there saw two to three new pediatric cases a week.