While countries like France, Holland and China already require sex education, few places demand that it be introduced at such a young age.
"It's vital that this information doesn't come from playground rumor or the mixed messages from the media about sex," Schools Minister Jim Knight said Thursday, announcing that sex ed would be added to the national curriculum.
English schools now are required to teach basic lessons on reproduction as part of the science curriculum. Northern Ireland, Scotland and Wales have separate education departments and standards. Only Scotland makes sex education voluntary.
The government hasn't detailed what the new curriculum will look like, but schools will be asked to provide lessons on relationships and contraception, topics not previously required. Lessons will become more sophisticated as kids get older.
Elementary schools can offer lessons in naming body parts, preparing for puberty and relationship feelings, Knight said.
For the very young, sex ed will mainly be about self-awareness, he said.
"We are not talking about 5-year-old kids being taught sex," he said. "What we're talking about for key stage 1 (ages 5-7) is children knowing about themselves, their differences, their friendships and how to manage their feelings."
But not everyone feels the state should decide when and how to broach the topic.
"I am not the parent who calls her son's penis a wee-wee. But I should decide if the word penis enters my child's vocabulary at 5 or not," said Elizabeth Talbot of London, who has two sons, aged 4 and 6 months old.
The government said children over 11 - the age at which kids generally go to secondary school in England - could learn how to develop respectful relationships and how risky sexual behavior contributes to the spread of sexually transmitted diseases and unplanned pregnancies.
Britain has among the highest teen pregnancy rates in Europe, with government figures showing that about 39,000 girls between ages 15 and 17 became pregnant in 2006, the year for which the most recent figures are available. An additional 7,200 girls between the ages of 13 and 14 were reported pregnant the same year.
Yet the country has long been considered more prudish and reserved than its continental neighbors.
"Everybody has sex at some point or other in their lives ... (but) we're not willing to prepare them," said Gill Frances, who served as part of a group that advised lawmakers on the new sex ed policy.
French students get sex ed in middle and high schools. Norwegian students typically get mandatory sex education around age 15. The topic is not mandatory but left up to principals and teachers in Italy.
In Finland, at age 11 or 12 children in school are taught about reproduction but sexual health and human relations begins between age 14 or 15. The Finish children's watchdog, the Mannerheim League for Child Welfare, last month proposed distributing free condoms to ninth graders (16 year olds) in schools to the stop spread of sexually transmitted diseases; education authorities rejected it.
In the United States, which lacks a national curriculum, the decision to offer sex education is left to individual states and districts. In recent years, the federal government has funded programs promoting sexual abstinence. The abstinence programs are favored by religious conservatives.
The topic has even made it into the presidential election campaign.
John McCain accused Barack Obama of being bad for families, saying he supports sex education for kindergartners. The legislation that Obama supported in the Illinois Senate would have required information deemed age appropriate, and Obama has said that meant warning young children about sexual predators.
In China, students generally begin sex education in middle school, but the curriculum is basic. In India, where more than 2.5 million people are infected with the AIDS virus, basic sex education has been offered in schools since the late 1980's. However, six of India's largest states banned the basic programs, saying they would corrupt the young.
Peru is implementing a pilot program this year in 146 schools that begins lessons on sexual reproduction at age 11 and on birth control at 14. It will be adopted nationwide next year.
"Statutory (sex ed) is absolutely crucial in reducing teenage pregnancy, particularly for vulnerable young people, but all children and young people need equipping with the skills and knowledge to help manage their lives," said Frances, chairman of the Teenage Pregnancy Independent Advisory Group.
Supporters of the government's plan for England say they hope the lessons give kids information they need.
"When parents fail to educate their kids properly, the government has every right to step in," said Gayla Coil, a Londoner and mother to two kids ages 13 and 10. "Me, I welcome the help."
Knight said teachers would get training and that schools would "ensure there is flexibility for schools to tailor lessons to reflect the values and beliefs of the parents and communities they serve." Schools will be expected to implement the curriculum by 2010.
Teachers will likely have mixed feelings about the change, said Nansi Ellis, head of education policy for a teachers union. "Some teachers will be uncomfortable with teaching this to very young children," she said. "But it's a really important part of children learning. Learning isn't just about academic achievements but the development of the whole person."
Associated Press Writers Elle Moxley in London, Anita Chang in Beijing, Nathalie Gentaz in Paris, Vladimir Isachenkov in Moscow, Victor Simpson in Rome, Doug Mellgren in Oslo, Norway, Matti Huuhtanen in Helsinki, Finland, Andrew Whalen in Lima, Peru and Gavin Rabinowitz in New Dehli contributed to this report.