Uganda, long regarded as one of the world's success stories in fighting the spread of the HIV virus, faces the challenge of continuing the progress it made in the 1980s and 1990s. One of the most important lessons of this period is that an important factor in preventing new HIV infections is the age of first sexual behaviour. When young people delay sex, they avoid early exposure to HIV. The human and public-health benefits of this postponement are enormous.

The United Nations stipulates "increased age of first sex" as a goal that all countries must aim for. Uganda's national strategic plan on HIV/Aids - guided by the country's Aids commission - has the responsibility of deciding what the new age targets should be. The commission's technical working group on prevention is now pondering this very serious question.

Cathy Watson is director of the Straight Talk Foundation and a senior Ashoka fellow

Also in openDemocracy's "HIV/Aids: what policy for life?" debate:

Donna M Hughes, "The 'ABC approach' to global HIV/Aids: good for women and girls"
(12 August 2004)

Tim France, "The United Nations and Aids: learning from failure" (30 May 2006)

Roger Tatoud, "Gendering the fight against Aids"
(21 August 2006)

Alex de Waal, "The global Aids campaign: a generation's struggle" (25 August 2006)

The experience of the past generation is revealed in statistics compiled in demographic and health surveys (in 1995 and 2000-01) and in the HIV/Aids sero-behavioural survey (in 2004-05); all showed that median age for first sex among Ugandan teenagers has indeed risen:

• for girls, the median age moved from about 15.9 (1980s) to 16.3 (1995), 17.3 (2000), and 17.9 (2005). An increase of two years is an impressive achievement, reflecting that girls have established some control over their lives

• for boys, the median age moved from about 17.5 (1980s) to 17.7 (1995), 18.3 (2000), and 19.1 (2005). The increase indicates that boys too have made smart strides.

It may be hard, however, to build on this progress. This is partly related to the relationship between sexual behaviour, education, and differences in the median age of marriage.

The average age girls marry in Uganda is 17.9. They have little premarital sex, and they are not likely to marry later unless they can stay in school longer. By age 19, only 12% of girls are still in school; 60% are having sex in marriage, 20% are abstaining, and 24% are having premarital sex.

The picture for boys is strikingly different. They marry much later, at about 22, and have a lot of sex before marriage. They also stay in school much longer. By age 19, about 50% are still in school; 6% are having sex in marriage, 35% are abstaining, and 59% are having premarital sex.

In principle, there looks to be plenty of room - especially where boys are concerned - for increasing the age of first sex. But here arises a problem. The 2004-05 sero-behavioural survey found that males who delay first sex until after the age of 20 are at increased risk of HIV.

Men aged 15-49 who started sex at 20 or above had an HIV prevalence of 6.6%, compared to 6.2% in men who had started sex between the ages of 18-19. Perhaps boys who start later are less skilled at choosing partners or negotiating sex? Whatever the reason, it might be unwise as a public-health measure to push boys as a group to abstain much past 20, although it can be an excellent strategy for individual boys.

What clearly does need a strong public-health intervention is early sex before the age of 15. This is currently increasing in boys (from just 10% of males now aged 23-24 to 18% of boys now aged 15-17) but decreasing in girls.

The above statistics may seem dry, but they represent millions of personal dramas as young people become sexually active in an environment with a heavy disease burden.

Who becomes sexually active, why and with who is - in Uganda as across the world - part of the social construction of sex. After the successes of the late 1980s and 1990s, changing the contours of sex and the HIV epidemic may now not be so easy. The hope must be that universal secondary education will gradually help Ugandan girls to postpone first sex, and thus contribute to progress in addressing the national and global scourge of HIV/Aids.