Children as young as 10 should be taught about contraception in their final year at primary school, a leading think-tank says.
Researchers say children should be taught about contraception sooner
The recommendation, by the Institute for Public Policy Research, comes after its study showed British teenagers are the most sexually active in Europe.
It also found the UK had the highest teen pregnancy rate while almost one in three 15-year-olds did not use condoms.
The Department for Education says the UK's teenage pregnancy rate is falling.
The report, Freedom's Orphans: Raising Youth in a Changing World, is released in full next month.
Currently all schools in England and Wales have to teach sex education to 11 to 14-year-olds as part of the science curriculum.
But schools can choose to teach it through Personal, Social and Health Education (PSHE).
The IPPR wants this to be mandatory.
Julia Margo, IPPR senior research fellow, said: "Over the last 50 years, the average age of first sexual intercourse has fallen from 20 for men and 21 for women in the 1950s to 16 by the mid-1990s.
"The proportion of young people who are sexually active before the age of consent has risen from less than 1% to 25% over the same period.
"Our education system must respond in kind and start teaching children about the risks involved in sex before they even consider taking those risks."
According to the IPPR report, in Britain there were an average of 26 live births per 1,000 women aged 15 to 19 - nearly a fifth higher than Latvia, the country with the next highest rate, and more than four times the rate of Cyprus, Slovenia, Sweden and Denmark.
Although the government's target is to halve teenage pregnancy rates between 1999 and 2010, there were 41.4 conceptions per 1,000 women under the age of 18 in 2005 - just 2.9 per 1000 lower than in 1991.
The study also discovered that over the last 10 years levels of genital chlamydia rose by 508% in teenage boys and 238% in teenage girls and genital herpes rose by 52% in teenage boys and 38% in teenage girls.
Meanwhile, levels of syphilis had increased significantly in teenage boys and girls.
The study included government figures and findings from several studies, including the Index of Child Wellbeing in the European Union, published earlier this year.
Anne Weyman, chief executive of the FPA (formerly Family Planning Association) said: "FPA has long campaigned for SRE to be mandatory and to begin at primary school.
"We welcome this report's findings. Starting age-appropriate education about sex and relationships at primary school gives children a solid base of information which is developed and improved on over time."
The Department for Education said that teenage pregnancy rates were at their lowest for 20 years.
A spokesman said: "We are taking steps to improve the support we give to parents to talk about sex and relationships, and we have made clear that local authorities and primary care trusts must make sure that they are providing young people with access to advice and contraception."