At What Age do Adolescents Become Sexually Active?
(Hint: It's later than you think)

Steven Mosher
by Carlos Polo
Population Research Institute
Weekly Briefing
11 July 2011
Reproduced with Permission

The age at which teenagers begin to engage in sexual activity is a critical variable for those who would make public policy in the health field. Early onset of sexual activity is associated with higher rates of sexually transmitted infections (STIs), teen pregnancy, depression, suicide, and other adverse consequences.

However, just how to determine this age can be difficult to determine, and some commonly used calculations can lead to grossly erroneous results, as a groundbreaking study by Dr. Jokin de Irala has now demonstrated.

Dr. Irala carried out a multi-national survey of 7,011 adolescents of both genders from private and public schools in Peru, El Salvador and Spain to gather data on the age of first sexual intercourse. In his study, which used a significantly larger sampling of adolescents than similar surveys, De Irala calculated the age at first sexual intercourse using both of the methods commonly relied upon: "the mean age at first sexual intercourse" and "proportions having already had sex at given ages."

It turns out that the second formula - the proportions having already had sex at given ages - shows that most adolescents are chaste for far longer than the sex educators, and the media, would have us believe.

"Average age at first intercourse, obtained from published epidemiological studies about sexuality, are often presented by the media in such a way that ignores the real meaning of the data. Those who rely upon the media for information, such as the general population and the young people themselves, can be misled," De Irala states.

What Dr. Irala is referring to is the claim, frequently repeated by the mass media, that "the mean age at first sexual intercourse is 15 years old." This claim frequently surfaces in the debates on how to prevent AIDS, when it is invariably asserted that the majority of adolescents at 15 years old have engaged in sexual activity. But this is simply not true. The majority are, the data show, still chaste at that age.

Of course, if public health officials believe that middle school-aged children are having sex, they will begin to push all kinds of programs in the schools that will have the perverse effect of encouraging children who are not sexually active to become so. This will have an adverse effect on the health of adolescents. In the case of AIDS, it could even be fatal.

Why is "mean age at first sexual intercourse" misleading?

It is misleading because it collapses the data into a single number. When someone reads that "the mean age at first sexual intercourse is 15 years old," they assume that the great majority of adolescents have had sexual intercourse at 15 years old. But this is simply not true. When you look at the complete data, as Dr. Irala has done, you realize that proportion of adolescents 15 years of age who have already had sex is only 20 percent!

The most common mistake made in the other studies that Dr. Irala examined is using only the individuals who have already had sexual intercourse at a given age to determine the mean age at first sexual intercourse. This approach results in a number that only describes the sexually active part of the population of adolescents. But if you consider the entire universe of adolescents - both those sexually active and those who are not - the results are very, very different.

De Irala's study demonstrates this very clearly. In the three countries he examined, "the mean age at first sexual intercourse" is about 15 years old. But now look at Table 1 below, the calculation of the "proportions having already had sex at given ages." Sexually active adolescents under the age of 18 are actually in the minority!

Table 1: Percentage sexually active at this age

In a personal interview, Jokin de Irala explained to PRI how even an expert can fail to appreciate how "median age of first sexual intercourse" greatly exaggerates the percentage who are sexually active at given ages. "Even most social scientists fail to appreciate this problem," he told us. "This slanting of the data should now be recognized for what it is, and this approach abandoned, by any researcher who wants to carry out a reliable investigation into adolescent sexual behavior," he added.

Public health organizations like HHS, USAID, UNAIDS, HRRS, etc., need to rethink their assumptions about the proportion of adolescents who are sexually active. Believing - wrongly - that the majority of adolescents are sexually active at 15 years old, they impose condom distribution schemes and sex education courses on kids at ever younger ages.

At the same time they refuse to consider abstinence as a viable sexual behavior because they are already convinced that "the majority does it." If these public health organizations recognized that in reality only 20% of 15-year-olds are sexually active, perhaps their priority would shift to strengthening the decision of the other 80% of adolescents to not have sex. (Or perhaps they wouldn't, since the goal of at least some of those who work in these organizations is to convince innocent children to have sex as early and as often as possible.)

Responsible public policy makers need to know that only a fraction of 15-year-olds are sexually active. They need to know that it is counterproductive, even dangerous, to distribute condoms and contraceptives to all adolescents. They need to know that such programs only produce more adolescents having sex in circumstances that put them at emotional, physical, and moral risk.

Spread the word.

  1. Source: De Irala J, Osorio A, Carlos S, Ruiz-Canela M, Lpez-del Burgo C. "Mean age of first sex: Do they know what we mean?" Arch Sex Behav 2011. DOI 10.1007/s10508-011-9779-4
  2. Available at (PDF).
  3. More articles by Dr. Jokin de Irala at: