"Modern life is increasingly complex and we have a duty to equip our young people with the knowledge and skills to deal with it," the minister said, announcing the move aimed at cutting high teenage pregnancy rates, sexually-transmitted diseases and drug and alcohol misuse.
At first, children will be given lessons about their body so that they can learn "about themselves, their differences, their friendships, how to have strong friendships and how to manage their feelings," Knight said, stressing that five and six-year-olds should not be "taught sex."
Later, children will have more detailed lessons, which will include contraception, abortion, homosexuality as well as the effects of drugs, the minister said adding: "It is right to share the responsibility between home and school." Personal, social and health education (PHSE) will be compulsory for pupils aged from five to 16.
"It's vital that this information doesn't come from playground rumour or the mixed messages from the media about sex," Knight said.
Wales has already introduced sex and relationship education in the school program along with Northern Ireland. Scotland has no legal requirement to teach sex education as part of the curriculum.
Britain has one of the highest levels of teenage pregnancy in Western Europe and the number of abortions carried out on teenage girls under 16 increased by 10% last year.