A surprising 80 percent of teenage boys say they are using condoms the first time they have sex, a government survey found in a powerful sign that decades of efforts to change young people’s sexual behavior are taking hold.

But another promising trend — a drop since the 1980s in the number of teenagers having sex — has leveled off.

Boys’ condom use may mean they are taking more responsibility for contraception or they are protecting themselves from sexually transmitted diseases, experts say.

Or, as one young man said, girls may be drawing the line.

“I’m not sure how much of this is guys thinking they need to use a condom or girls insisting they use a condom,” said 17-year-old Olivier Vanasse of Princeton, N.J. “I’d be hesitant to give guys credit for coming up with this on their own.”

The study, released Wednesday, is based on interviews with about 4,700 teenagers, ages 15 to 19, conducted from 2006 through 2010.

It shows the percentage of boys who said they used condoms the first time they had sex climbed from 71 percent in 2002 to 80 percent in the new survey. In 1988, 55 percent of boys said they used a condom during their first sexual intercourse.