More than 40% of American teens ages 13 to 17 say they’ve gambled in the past year, according to a survey published Wednesday, raising red flags about the popularity of betting among minors.

Researchers from Rutgers University’s Center for Gambling Studies surveyed nearly 1,700 Americans between 13 and 27, and found that 52.5% of the respondents said they had gambled. Among those 17 and under — for whom gambling is illegal — the share was nearly 43%.

“We are living in a time where anyone can have a casino or sports book in their pocket,” said Lia Nower, the director of the Rutgers center and lead author of the new report. “Now teens who should be playing sports are betting on them, often with family members.”

Nower and her colleagues conducted the survey over the spring and early summer, describing the effort as the first national gambling prevalence study of U.S.…