The finding comes from a survey of 1,373 British teens by Bethan Hatherallof London's National Children's Bureau, and colleagues.
Does that mean they are protected against sexually transmitted diseases? Not necessarily. Hatherall and colleagues find that 6% of the kids (who had reported that they used a condom the last time they had sex) said they put the condom on after vaginal penetration -- and 6% said they continued vaginal penetration after condom removal.
Diaries provided by 74 of these sexually active kids gave the researchers a closer look. As it turns out, nearly a third of the kids put condoms on too late at least one time during the six-month diary period. And nearly one in 10 took them off too soon.
This means that simply telling teens to use condoms isn't going to help many of them avoid spreading sexually transmitted diseases.
"The reduced effectiveness of condoms as a method of sexually transmitted disease prevention when used incorrectly may result in users losing confidence in what should be a highly effective method," Hatherall and colleagues conclude.
The findings appear in the early online edition of the journal Sexually Transmitted Infections.