By Gary Eason
BBC News website education editor
How much impact does starting young ultimately have?
Spot the difference:
"Children of five to be given sex education" - newspaper headline.
"We are not suggesting that five and six-year-olds should be taught sex" - Schools Minister Jim Knight.
The difference may be semantic but it illustrates the problem.
Sex is regarded as such an important but sensitive subject that any government which starts dictating what children should or should not be taught risks offending someone.
One woman's vulva is another's "down below".
Children in some primary schools know the correct name, as part of their lessons on the differences between males and females.
No, not clothing necessarily, nor hair length, skin colour, the shape of the nose or even their names.
"Women have boobs and boys don't," says eight-year-old Malachai, to the amusement of his classmates at a primary school in London.
"Right," says the teacher, "get it off your chests, laugh it off."
She then praises him - "someone had to say it" - but acknowledges the embarrassment of some of the children and steers them professionally onto the correct terminology.
Thus "females have breasts" joins the list of differences on her flip chart.
"I think it's really important," the teacher says.
"Education is key. We've got the highest teenage pregnancy rate in Europe - sexually transmitted diseases on the increase.
"So if we start now, bit by bit - you saw that in that lesson, there was nothing mentioned about 'having sex' - step by step in appropriate ways ... by the time they get to secondary school they have got some knowledge".
Parents often do not talk about these things, she says.
"But Nature as it is, whether you want to talk about it or not, things will start happening."
And this is the theory underpinning the government's decision to make PSHE lessons - personal, social and health education - compulsory in all England's schools.
What has not been published yet to underpin the decision is the actual content of the curriculum, particularly in primary schools where provision is currently most patchy.
The Department for Children, Schools and Families said a London head teacher, Sir Alasdair MacDonald, would lead a further review into how best to make PSHE compulsory, "ensuring that there is a place in the timetable and flexibility in the curriculum to take schools' ethos, pupils' needs and parents' values into account".
It added: "Updated guidance will also be produced covering the content of the curriculum, based on the existing non-statutory programme of study."
The department would like this programme - covering, among other things, same-sex relationships and contraception - to be "consolidated".
Meanwhile, Sir Jim Rose will look at how PSHE should best be delivered in primary schools as part of his ongoing review of the whole primary curriculum.
Ministers have been saying they want the emphasis to be on sex and relationships.
And research suggests that may be where the most lasting benefit of any changes will lie.
For the idea that more, high quality sex education leads to fewer teen pregnancies is not borne out.
One of the most authoritative studies of the issue, conducted in Scotland, looked at the impact over five years of a sex education programme called Share.
This differed from more conventional sex education by having intensive teacher training and a focus on developing skills rather than simply providing information and discussing values.
Lead researcher Marion Henderson said: "Unfortunately, it shows that pregnancy and abortion rates in the girls who were taught using the enhanced programme were no different from rates in the control group who were given conventional school sex education."
What positive results there were lay in the improved quality of relationships reported by those who had taken part.
This is an important consideration, given the negative effects of family breakdown, but is not what ministers hope to see from better sex education in England.
Gill Frances, chairman of the Teenage Pregnancy Independent Advisory Group, said: "Statutory PSHE is absolutely crucial in reducing teenage pregnancy, particularly for vulnerable young people, but all children and young people need equipping with the skills and knowledge to help manage their lives."
But a later study of almost 5,000 pupils in 24 schools looked at how factors outside the classroom might influence their sexual behaviour.
The upshot was that the socio-economic characteristics of the neighbourhood were the main influence: the higher the deprivation, the more likely they were to be having sex at an earlier age.
"What the results tell us is that to make an impact on levels of early sexual activity and pregnancy the government would need to tackle deprivation and socio-economic problems within neighbourhoods," Dr Henderson said.
In a review carried out in North East Lincolnshire, which has a particularly high rate of teenage pregnancy, poor sex education was only one of the factors found to be at play.
At least as important were local culture, including poor self-image; substance misuse and economic factors, especially deprivation, councillors were told.
"In most cases the mothers of the teenage girls have also given birth at an early age," the teen pregnancy report said.
"There is a trend that girls at the lower end of the academic scale may see having a child as the only option, as schools focus and are measured on A*-C grade GCSE pupils."
Pregnancy was often perceived by a young person as an achievement or seen as a way out.
"There are young people who are actively trying to get pregnant, who admit that the reason for this is lack of family attachment."
As with so much else in the education system, the key factor may be poverty - including poverty of aspiration and all that goes with it.