Children as young as three should be taught about sex and fertility before starting at primary school or risk future chances of parenthood through a lack of knowledge, experts have warned.
Young adults receive only 20% of information about fertility from official sources and often do not learn about their reproductive health until they are trying to conceive, it was revealed.
The comments were made at the Fertility Health Summit, hosted by Royal College of Obstetricians and Gynaecologists, a conference of leading fertility experts coming together to address how to improve young people's knowledge of fertility and reproductive health.
They advocated a German or Swedish scheme, applied from an early age, using games and social media to engage with young people on the topic.
Professor Joyce Harper, who works for the Embryology, IVF and Reproductive Genetics Group at University College Hospital London, advocates teaching sex education and fertility from a young age, and said "all boys are obsessed with their penises".
She said: "We need to start at primary school ... maybe even younger.
"Yes, kids are going to ask about their anatomy.
"I have two boys ... they asked about their penises very early.
"All boys are obsessed with their penis, so it's a good time, as a parent, to follow that on."
Professor Adam Balen, chairman of the British Fertility Society, who organised the summit, said: "We should look at three and four-year-olds being introduced to ideas about where babies come from."
A new poll, conducted to mark the summit, found that many 16 to 24-year-olds found "worrying gaps'' in their knowledge of fertility and reproductive health.
While fertility rates for both sexes decline gradually from the late 20s, and can be affected by genetic and environmental factors such as smoking, obesity and nutrition, four in five of 1,000 surveyed thought a women's fertility only started to decline after the age of 35.
Professor Balen said 15% of people were affected by infertility, and added: "We now know that more women are delaying their childbearing and having children over the age of 35 than under the age of 25."
He attributed this to social pressure, young women wanting a career, economic disadvantages of women and the lack of commitment from men.
The study also found the majority of young adults got their information on fertility from the internet, their friends and family and social media.
Professor Harper also stressed more effort should be made to teach online, through Youtube and social media.
She said: "We have to appreciate how young people communicate and gain information.
"As an educator, I'm very aware that an added teacher ... is Google or Wikipedia.
"Children are going to learn about sex here, about fertility, about pregnancy, all of these things, using social media.
"The trouble is there is some really bad information on social media, apps and other digital forums, so I think this is an area where we really have to try to get with it and put some good information out there."