A federal law requiring pornography producers to verify performers are at least 18 years of age withstood an industry challenge Thursday after a federal judge rejected arguments the measure imposes burdensome record-keeping requirements that chill free speech.

Forcing producers of sexually explicit content to maintain records showing that performers are adults is reasonable given Congress' interest in combatting child pornography, said Philadelphia-based U.S. District Judge Michael Baylson, citing evidence that porn purveyors are profiting greatly from images that depict young-looking performers.

"Youthful-looking performers are ubiquitous in the adult entertainment industry and there is a significant market for pornographic materials depicting such individuals," he wrote in his ruling, which noted the fascination with youth is hardly a recent phenomenon and cited Dante, Mozart's "The Magic Flute" and Vladimir Nabakov's "Lolita" as examples.

Under federal law, producers of sexually explicit photos, videos and other media must obtain government-issued photo ID from each performer and keep copies on file for government inspection. The law also imposes labeling requirements.

The Free Speech Coalition filed suit in 2009, arguing the law is overbroad and tramples on protected speech. The industry said it already takes measures to keep kids out of porn.

The government presented evidence that 500,000 Internet searches a day — at least a third of the total for sexually explicit material — involve "teen porn" and similar keywords. Such searches have grown more than 200 percent over the last decade, the evidence showed.

"Without a strong and robust regime for policing the ages of performers, there would be incentives on both sides of the equation to use minors as performers in sexually explicit depictions," Baylson wrote.

For porn producers, he wrote, "there is a high commercial value associated with the young-looking performer." He said the record-keeping requirement is further justified because "there are undoubtedly minors who do not contemplate becoming doctors, lawyers or hedge fund zillionaires, and would be attracted to the financial benefits available in the adult pornography industry."

The industry plans to appeal, saying Baylson's ruling provides "a strong basis for reversal and the ultimate dismantling" of the age-verification law.

Baylson first dismissed the industry's lawsuit in 2010. The 3rd U.S. Circuit Court of Appeals, while upholding most of that ruling, ordered him to take another look at several of the porn purveyors' claims.

Gail Dines, a Wheelock College sociology professor who testified for the government, called the age-verification law an important tool for keeping minors out of commercial pornography, given the soaring popularity of adult sites featuring young-looking performers.

Without the record-keeping requirement, she said Thursday, "how do we know that woman who is supposed to be 18, but who is acting like a 14-year-old, isn't actually 14? We would have no way of knowing that."

She argued the porn industry is "chipping away at societal norms that say adolescent and prepubescent females are off-limits to adults."

In a victory for the industry, Baylson found a provision allowing for unannounced, warrantless inspections to be unconstitutional because it did not provide for advance notice. But he declined to issue an injunction against the government, calling the issue moot because the FBI's inspections program has been shut down since 2008.