DENVER -- Colorado officials have cleared the language of a proposed ballot measure that would establish the nation's first legal limits on buying smartphones for children.

Backers of the move to forbid the sale of smartphones to children younger than 13 would now need about 300,000 voter signatures for the proposal to make the 2018 ballot.

The ban would require cellphone retailers to ask customers about the age of the primary user of a smartphone and submit monthly reports to the Colorado Department of Revenue on adhering to the requirement.

Retailers who sell a phone for use by a youngster could be fined $500, after a warning.

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The campaign is being led by Tim Farnum, a Denver-area father and doctor. Farnum is an anesthesiologist and Founder and President of Parents Against Underage Smartphones.

"We see this in the same light as maybe alcohol or cigarettes," he told CBS Denver in May.

Farnum, the father of five children ages 19 to 11, views children constantly on smartphones as addiction to an electric pacifier.

"The kids aren't playing out there anymore," Farnum told CBS Denver. "There are certain critical stages of development that are not happening."

Democratic state Sen. John Kefalas said he understands the reasoning behind the proposed law. But he told The Coloradoan that it would overstep the government's role.

"Frankly, I think it should remain a family matter," he said. "Ultimately, this comes down to parents... making sure their kids are not putting themselves at risk."

Last fall, the American Academy of Pediatrics released new guidelines for children's media use, including smartphones. The doctors recommended restricting screen time to no more than an hour a day of high-quality programming until age 6, after which parents should set consistent time limits and make sure electronic devices don't take time away from sleep or physical activity.