The charities say the move would reduce pregnancies and diseases
Children as young as four should be given compulsory sex education, two leading sexual health charities say.
The Family Planning Association and Brook told BBC Newsbeat more should be done to cut abortion rates and sexually transmitted infections among teenagers.
They argue gradual education from such a young age would help children not to rush into sex when they were older.
The Department for Children, Schools and Family said it was reviewing the
delivery of sex education in schools.
Children aged four might be taught about the names of body parts and basic ideas about different relationships.
The government is not giving young people enough information about sex and relationships, the charities add.
Brook chief executive Simon Blake said: "Many young people are having sex because they want to find out what it is, because they were drunk or because their mates were.
"That's just not good enough for young people. We've got to have high expectations for them so they've got high expectations for themselves."
He added: "All the evidence shows that if you start sex and relationships education early - before children start puberty, before they feel sexual attraction - they start having sex later.
"They are much more likely to use contraception and practise safe sex."
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The basic sex education that children are given in science classes does not go far enough, the charities say.
They want sex and relationship education on the curriculum across the UK alongside other compulsory subjects such as maths and English, as is the case in Northern Ireland.
The DCSF said effective sex and relationships education is essential for young people to make safe and healthy choices about their lives and prevent early pregnancy and sexually transmitted infections.
A spokesman added that an advisory group on the issue would be making recommendations on new policy to the government later this month.
Sixteen-year-old Bethany, from Norwich told BBC News she had not understood the consequences of having sex early on.
"I didn't know I could get pregnant," she said. "I think if they started introducing sex education a bit earlier and teaching us a bit more about it so that we were more aware it would have helped me a lot."