Acknowledging, however, that the announcement raises "complicated issues," the government has appointed a headteacher to carry out a review into how the new proposals can be best implemented, given local circumstances and parents' values.
The new policy will apply to 6.5 million schoolchildren in 22,500 schools in England, a government spokesman told AFP.
"This is a bold move and a necessary one," said England Schools Minister Jim Knight.
"Modern life is increasingly complex and we have a duty to equip our young people with the knowledge and skills to deal with it."
In a statement, the Department for Children, Schools and Families said that Downing Street "sees education as key to supporting young people to avoid teenage pregnancy, STI (sexually-transmitted infections), drug and alcohol misuse."
The statement said that ministers recognise that parents had to take the lead, but noted that schools could help by providing information and emotional and social skills for pupils.
Collated figures for England, Scotland, Wales and Northern Ireland show one of the highest rates of teenage pregnancy in western Europe, and critics of the new policy warned that the proposals could raise that even higher.
Christian Voice's National Director Stephen Green said the plans would "encourage experimentation," and added that teaching young children about sex was "a wickedness" from ministers who wanted to see "a whole generation fornicating."
The lessons will be adapted to suit the target age group, with children as young as five learning that animals give birth to offspring, naming body parts and preparing for oncoming puberty.
When it comes to drugs, primary school students will learn about how medicines and other toxins affect the body, while older pupils will study drug and alcohol misuse, the laws surrounding those issues and their social impact.
Education is governed separately in England, Scotland, Wales and Northern Ireland.