The study, published online in the journal Pediatrics, found 0.6 percent of 10-year-olds, 1.1 percent of 11-year-olds and 2.4 percent of 12-year-olds said they had sex. However, sex among very young adolescents was frequently involuntary, the study found.
The study also found 33 percent of those age 16, about half of those age 17, 61 percent of 18-year-olds and 71 percent of 19-year-olds said they have had sex -- a pattern that has prevailed for decades.
Lawrence B. Finer and Jesse M. Philbin of the Guttmacher Institute in New York said a low level of sexual activity among young adolescents has long been the norm, while sexual initiation later in adolescence has been and remains a normal part of teens' development process.
At the same time, some teens have delayed starting sex; in the current cohort of adolescents, the likelihood of sexual activity at any given age was lower than at any time in the past 25 years.
"Policymakers and the media often sensationalize teen sexual behavior, suggesting that adolescents as young as 10 or 11 are increasingly sexually active," lead author Lawrence Finer said in a statement. "But the data just don't support that concern. Rather, we are seeing teens waiting longer to have sex, using contraceptives more frequently, and being less likely to become pregnant than peers of past decades."
More than 80 percent of 16-year-olds used a birth control method at first sex, and one year following first sex, 95 percent used contraceptives.
The study authors used newly available public data on sexual initiation -- defined as vaginal intercourse -- contraceptive use and pregnancy among U.S. adolescents ages 10-19 from the 2006-10 National Survey of Family Growth, conducted by the U.S. National Center for Health Statistics and other sources.