And while girls with a serious older boyfriend by seventh grade were twice as likely to have had sex as those who'd had a same-age boyfriend, no such pattern was seen between boys and older girlfriends.
However, those couples were rare, which might make it hard to spot trends in that group.
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Among boys, those with a serious girlfriend by seventh grade were twice as likely to have had sex in the year before the ninth-grade survey -- compared with the girls, who were three times as likely.
About 6% of the boys reported having a girlfriend that much older than they.
In the ninth-grade survey, students were asked, "Have you had sex in the last 12 months?
Girls who hit puberty early (menstrual periods by sixth grade) were more likely to have an older boyfriend by seventh grade.
"This finding is of concern because young females are experiencing menarche (the start of menstrual periods) at increasingly early ages," the researchers write.