Today more than 50 per cent of young people worldwide are sexually active by the time they are 17. Many start younger not just because of earlier onset of puberty but also because of "a globalised youth culture" promoted by satellite television channels and the internet, says Jo Reinders of the World Population Foundation.
The increasingly sexualised youth culture talks of enjoyment and pleasure and draws young people in. Girls still get information about sex from magazines and friends, but many boys get their only information from porn sites, says Mr Reinders.
"The pill in the 1960s was the first sexual revolution, the internet is the second sexual revolution," says Gill Greer, director general of the International Planned Parenthood Federation.
"Many young people just want simple, straightforward answers. If adults give up on sex education, they will go to the internet."
Even in liberal Holland where sex education is taught at an early age, the internet is leading to changes in sexual behaviour, including a rise in anal sex, Mr Reinders said. "Soon sex education will not be about countering ignorance but changing attitudes formed by going to pornographic sites which degrade women."
The most needy area is in Africa and Asia where the HIV and Aids pandemic needs to be countered. Donor-funded programmes allow sex education curriculums to be drawn up, but, says Tania Boler of Actionaid: "Sex education never gets taught. Teachers simply do not have the time or training to do it."
Schools in India will teach children as young as five about sex, health and drugs from next year. The sub-continent has the world's highest national HIV and Aids caseload: 5.7 million people, it is estimated, are infected.
Ashok Ganguly, an education ministry official, admits its introduction will be tough, especially in rural areas where talking about sex is taboo.
It may be the world's second largest economy but its record on sex education is poor. The subject is not on the curriculum and the issue is so sensitive that teachers speak about it in fear of losing their jobs. For the majority of elementary schools, sex education has meant at most an hour each year devoted to what was previously known as "purity studies".
Recently, the government called for a more "moralistic" type of education.
This would not even include teaching the rudiments of sex education.
A former prime minister even went so far as to decry schools with corrupting the nation's young with "too explicit" lessons. The culprits? Anatomically correct scientific models designed with teaching pupils in mind.
Meanwhile, the young are free to peruse pornographic comic books which display images of rape and sex. One survey showed most 15-year-olds get all their information from friends, television and comics.
In spite of parliamentary proposals to make it compulsory, the topic is still non-curricular. In the absence of government guidelines, schools can decide for themselves whether or not to offer lessons. Many do with the highest concentration of courses in the industrialised north.
Lessons are sporadic. Pupils in upper secondary schools (15 to 19-year-olds) may have to make do with a single visit from an outside expert who is usually from the local health authority. Aids prevention and contraception are likely to be the focus.
The Catholic church believes that sex education should only be approached from a moral viewpoint. In some schools the only sex education comes from teachers of religion vetted by the diocese.
Sex education here is determined largely by individual states or schools districts. Accordingly, provisions vary widely and many states allow parents to take their children out of the lessons. According to Martha Kempner, of the Sexuality Information and Education Council, it is unusual for pupils to receive sex education before middle or high school (age 12).
Lessons are often organised around specific issues such as prevention of HIV, sexually transmitted diseases and teen pregnancy.
The Bush administration has controversially presided over a rise in funding, from pound;50million in 2000 to a projected pound;113million next year for abstinence only programmes teaching that no sex before marriage is the only sure way to avert teenage pregnancy or HIV transmission. It avoids discussion of contraception.
A 2004 Harvard university poll found that a third of heads at US middle and high schools offered abstinence-only programmes. An audit, in the same year, by Democrat congressman Henry Waxman found them riddled with misinformation. Many disparaged the effectiveness of contraception.
Here it is a compulsory part of the health education programme. Though centred on biology, the concept is much wider than the mechanics of reproduction. It aims to teach "responsible behaviour, respect of oneself and others". Teachers are expected to include themes throughout the curriculum, in subjects such as literature, civics, art and history.
The education ministry states the aims of sex education are to identify different aspects of sexuality, biological, emotional, cultural, ethical, social and legal. Pupils should be told where they can find information, help and support inside and outside the school.
From about 13, pupils start learning about prevention of unwanted pregnancies and sexually transmitted diseases.
Two decades after the first case of Aids was diagnosed, the Kenyan government in 2000 introduced a weekly compulsory sex education lesson for all pupils in primary and secondary schools countrywide. However, the syllabus met hostility from conservative religious groups who said it would lead to an increase in teenage pregnancy and the spread of Aids. Most teachers said they were not prepared to teach it and others believed it was responsible for early sexual activity.
According to a survey carried by Kenya's National Union of Teachers, only 45 per cent of staff know Aids has no cure. About 25 per cent thought herbs and traditional medicine could cure the infection while another 13 per cent said witchdoctors could treat the disease. More than 10,000 teachers have died from the condition in the last five years.
Most pupils said they get more information on Aids from electronic media than from teachers.
At one time the country had the unenviable reputation of having the highest HIV infection rate in the world, but it was also the first in sub-Saharan Africa to report a dramatic decrease from more than 25 per cent infection rate in 1980s to 6 per cent in 2002. The decline was achieved through a rigorous nationwide campaign that included condom distribution, popular songs, soap operas and support services.
It resulted in a huge drop in the number of pregnant women with HIV. The use of condoms among men aged 15 to 19 also increased substantially from 20 per cent in 1989 to 60 per cent in 1995. Unfortunately, those gains might soon be lost. Uganda has redirected its Aids prevention towards a political programme promoting sexual abstinence until marriage.
One of the most Catholic countries in Latin America, it will introduce sex education in all state schools for children aged four to 15 for the first time next year.
Milta Fernandez Trevi$o, deputy general secretary of the Union for Educational Workers, said: "It covers the sexual processes and how a child should care for their body, understand physiological changes, and be aware of potential abuse. Nursery school children will just learn how to care for their bodies. State schools have no responsibility to provide sex education, although some private schools do during biology lessons.
All teachers would be expected to teach sex education. However, devoutly Catholic teachers who feel that sex education conflicts with their religious views will be able to opt out.
Alex Benedikz and Sam Haidari, both 13, attend Stockholm's Engelska Skolan Norr, and have been able to get free condoms from the school nurse since they were 11 years old.
"She showed a cartoon with people having sex and talked about Aids and other diseases," said Sam. "The nurse said we can ask her for free condoms.
So I got one to see if it was true."
To ensure all children are aware of the facts of life, sex education is integrated into the national curriculum for all 11-year-olds.
In a country where most people have a matter-of-fact attitude to nudity and discussions about sex, schoolchildren are encouraged to talk openly about sex and relationships in school.
At senior high school, sexual education continues with discussions about sexual identity and sexual orientations. "We invite representatives from gay, bi-and-transsexual organisations to speak to students," says Alex's mother, Dr Margret Benedikz, headteacher at Stockholm upper secondary school, IEG.
Reports by Michael Fitzpatrick, David Newbold, Suchitra Behal, Stephen Phillips, Jane Marshall, Wachira Kigotho, Jason Mitchell and Jon Buscall