In years past, 12 was considered the lower end of the age range for the start of blackheads and whiteheads. With earlier onset of adrenarche (when the adrenal gland awakens) and menarche (first period), the authors of the guidelines suggest, “there appears to be a downward shift in the age at which acne first appears.”

“I’ve definitely seen a shift,” said Dr. Latanya T. Benjamin, a dermatologist at Lucile Packard Children’s Hospital at Stanford, who did not help draft the guidelines. “It’s not uncommon for a 7- or 9-year-old to walk in with the first signs of acne.”

But whether children are experiencing early, or precocious, puberty has been the subject of scientific debate. A more likely cause of the increase in cases, some experts say, is that parents are less tolerant of acne and doctors more willing to provide powerful acne treatments to children.

The research re-energizes a century-old treatment method that was abandoned with the rise of antibiotics during World War II. As germs have built up a resistance to those drugs in recent years, scientists are seeking alternatives and the virus strategy “is in vogue again,” said Vincent Fischetti, a biologist at Rockefeller University in New York who is one of the pioneers of the revived approach.

The study of the acne-fighting viruses, called bacteriophages or simply phages, was published in the September- October edition of mBio, the journal of the American Society for Microbiology.

In it, scientists found phages that live side-by-side with the P. acnes bacteria on the faces of people who don’t get bad acne, theorizing that the viruses somehow helped to keep it under control, said Laura Marinelli, the lead author and a postdoctoral fellow at the David Geffen School of Medicine at the University of California, Los Angeles. The single-celled P. acnes bacteria that resides in pores can grow out of control in an oily environment.

Once they identified the viruses, the scientists found the viruses had the ability to kill isolates of the bacteria in lab dishes, opening the possibility they may one day be the basis for effective treatments for the most common skin disorder in the U.S., with more than 40 million sufferers.