Why delaying sex may be important

On a population level, delaying a young person’s sexual debut, or at least the age at which they start having full intercourse, could be a very effective HIV prevention measure in certain groups.

In the developing world, young women are much more vulnerable to HIV than young men. In southern Africa, for instance, HIV prevalence among young women aged 20 to 24 is two to three times higher than among men of the same age. Without this huge peak in HIV incidence in the late teens amongst young women, HIV prevalence in the general population would be much lower.^{1 2}

The above figure is from Swaziland, 2009,² but could apply to a number of other countries with generalised epidemics. The period of peak HIV incidence in young women, when the increase in prevalence is steepest, occurs between the ages of 15 and 24. The period of steepest HIV incidence in men lags behind by five to ten years.

Recent research suggests that this is not because young men simply start sex five years later and have sex with partners who are five years younger (the ‘sugar daddy’ hypothesis). Instead, young women and young men start sex about the same time. But the younger age limit of young men’s partners increases much more slowly than the older age limits. So while 15-year-old boys who were sexually active in the Swaziland survey rarely had sex with girls outside the ages of 13 to 17, 25-year-olds were in the main having sex with partners between 16 and 25 – a wider age range (see below).

This diagram, from the same study, plots the age of 872 young men aged 15 to 24 against the age of their female partners at the time they started having sex. The upper solid line is the line of age equality between the sexes. The lower solid line plots the actual average age of the female partners of young men at a certain age. Thus, while the average age of the girlfriends of 17-year-old men was 16, the average age of the partners of 24-year-old men was 20.

The inner dashed line is what the 95% confidence interval of the ages of female partners would be if the spread of ages of female partners remained the same as men got older (in other words, only 5% of female partners would have ages outside these lines). The outer dashed lines are the actual 95% confidence intervals, showing that the spread of age of young men’s partners increases as the men age.

By the time they are 25, men have entered their own period of maximum HIV incidence, so their younger female partners will therefore be very vulnerable to infection. Research has also shown that men who show a pattern of preferring younger partners also have more partners and take more sexual risks. Young women are also more vulnerable to HIV biologically: there is evidence that the immature genital tract of young women is more susceptible to HIV and other sexually transmitted infections (STIs).

Whatever the reason, this has contributed to a situation in which teenage girls in southern Africa are five to ten times more likely to have HIV than teenage boys.

Abstinence is most commonly discussed in African and American contexts, but another population among whom delayed debut of intercourse might serve to reduce HIV infections is young gay men.

In the 2002 UK Gay Men’s Sex Survey,³ the authors comment that while the median age for sexual debut for gay men is two years later than that of heterosexual men (16, as opposed to 14), gay men’s median age for first penetrative intercourse is in fact earlier (17, as opposed to 18).

Unfortunately, lesbians and gay men are largely excluded from abstinence education programmes – explicitly so in the case of ‘abstinence only till marriage’ in countries and states that do not allow same-sex marriage or civil partnerships.