It's a conversation parents dread having with their children, with many being happy to avoid it for as long as possible. 

But mums and dads should be talking to children about sex from the age of three to avoid them growing up thinking the topic is off-limits, according to Professor John Ashton, The Sunday Times reports.

Professor Ashton, president of the Faculty of Public Health, says parents should use questions from their children as an opportunity for biology lessons - even at a tender age. 

If a young boy, for example, asks why his anatomy is different from that of girls, parents should respond with factual information about sperm production. 

And he warns that if parents choose not to have these conversations, by the time their children are teenagers and need advice they will seek it elsewhere.

He said: 'Part of the problem is that parents avoid questions about, "why is he different from me?" or "why is she different from me?". 

'You have to put the building blocks in place for the serious conversations that young people need to have when they reach their teenage years,' he said. 

'If you don't try to answer your children's questions factually, or you avoid them, they stop asking them and by the time you get to 13 and 14, that's it, they don't tell.'

He warns that if children don't understand the biology of sex from a young age, they will not be well prepared to deal with complex matters in relationships when they are teenagers, such as consent and practising safe sex.   

Currently, pupils in UK schools don't start sex education until Year Seven, the first year of secondary school when they're aged around 11.

But Professor Ashton believes sex education should be compulsory in all primary and secondary schools.

He also suggests that parent teacher organisations get involved in advising parents about how to raise these matters with their children. 

The Professor has previously called on the government to consider lowering the age of consent from 16 to 15. 

He argued that the current limit prevents sexually active teenagers seeking support from health authorities with contraception and sexually transmitted diseases.

In February 2015, MPs said that children should receive compulsory sex and relationships lessons in primary school, MPs have said.

Youngsters have a right to information that will help to keep them healthy and safe and should be taught the subject throughout their schooling, according to the influential Commons education select committee.

In a new report, it called for personal, social, health and economic education (PSHE) to be given statutory status in all of England's state primary and secondary schools to ensure that enough lesson time is devoted to the subject and teachers are properly trained.

Sex and relationships education (SRE) should be a 'core part' of these classes. 

The latest official government guidance on SRE is 14 years old, and many people told the committee that the world is now very different - seeing changes such as the rise of social media, and new laws on same-sex marriages.

Some noted that increasingly easy access to pornography through the internet is shaping young people's behaviours - such as "sexting" and fuelling the need to reassess PSHE.

Written by Siofra Brennan

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