The closest I came to sex education at school was the story of a teenager called Mary. She was already pregnant when she married a young man called Joseph, even though he wasn't her baby's father. Mary's baby was called Jesus. The western world has just celebrated the 2007th anniversary of his birth. This serves to remind us that not all teenage pregnancies are a cause for despair.

Most are, however, at least a cause for concern. Few teenagers are financially independent, so the state ends up providing for them and their offspring. When girls as young as 12 and 13 become pregnant and give birth, it robs them of their childhood, narrows their opportunities and offers a poor start to their baby. So, with teenage pregnancies approaching 10,000 a year in Scotland, a senior public health doctor has called for sex education in schools to start at five years old. It's a suggestion the Catholic Church dismisses as "throwing petrol on a fire".

Ask most adults about it, and they will instinctively say that five is too young for sex education. "Let them have their childhood," people will reply, or: "Don't spoil their innocence." It's my instinct, too, yet I believe there is an important distinction between ignorance and innocence.

When I was five years old, I was firmly told that the district nurse brought babies in her Gladstone bag. Today's small children proudly point to the new baby in their (or their friend's) mummy's tummy. They are no less innocent for knowing where it is or how it got there.

But surely at that young age this knowledge is something that should be passed on at home - and by all means referred to at school - but without the emphasis of a curriculum-guided lesson. To start formal sex education from so young an age, we would need to know it was a proven way of preventing teenage pregnancy. And we don't. On the evidence we have, preventing teenage pregnancies seems to be less about starting sex education young than about involving home and community when it does begin.

Britain has 50,000 teenage conceptions a year, topping the European league. (Some 47% of 16-17-year-olds and 58% of younger girls then opt for abortion.) Westminster committed £150m in 1999 to halving the numbers south of the border by 2010, but in a pre-Christmas statement admitted it had no hope of success.

To date, the percentage has dropped by only 11 points. Meanwhile, the number of teenagers has risen, commensurately increasing pregnancy numbers from 46,655 in 1999, when the initiative started, to 47,277 in 2005. This rise has happened despite policies that include providing contraceptives and the morning-after pill in schools, without the consent of parents.

Contrast that with elsewhere in Europe. In the 1970s, Italy, France and Holland had similar teenage pregnancy rates to Britain. In Britain today, it is four times higher than Italy, three times higher than France and six times higher than the Netherlands. The Netherlands is bottom of the European league with 8.4 pregnancies per 1000 teenagers. It is, therefore, to the Dutch we should be looking for example.

Half of the Netherlands' primary schools and almost all its secondary schools offer sex education. This focuses on the biological aspects but includes values, attitudes, communication and negotiation skills. The approach is one of openness and, thus far, it probably diverges very little from what goes on here. But in the Netherlands the teaching is echoed throughout society. They've had prime-time television talk shows; health care is confidential, anonymous and non-judgmental. And, crucially, parents take an open and pragmatic approach with their children. They accept that teenagers will have a sexual relationship at some point and are dutiful about preparing them for the responsibilities that go with it. Responsibility is the key word throughout. The upshot is that Dutch teenagers usually don't have sexual intercourse until they are almost 18.

Clearly, there is a lesson to be learned. But the lesson is not that we should start sex education at five. It is that we should follow through what is taught in schools by offering a consistent approach at home, in the community and in the media. As things stand, we have separated school and home. School offers information and advice (and, in some cases, contraception and the morning-after pill, without parental knowledge or consent).

The trouble is that school is an island in a sexualised media, a drinking culture and with parents who abdicate their roles. Meanwhile, kids are just kids. They are still driven by hormones and peer pressure more than by reason. So sex education plus freely available contraception seems to have culminated in permission to have sex with, or all too often without, the necessary protection.

The rise in sexually transmitted diseases, as well as pregnancy, demonstrates that the current strategy is ineffective. So, on what basis should Scotland now extend sex education to five-year-olds? Instead of compounding the felony, surely what is required is a rethink.

There is another interesting statistic. In relatively well-off areas, progress has been made in cutting the numbers of teenage pregnancies. The message teenagers need to receive from parents and from society is that they are far too precious and have way too many prospects in life to be saddling themselves with a baby. It also happens to be the truth. This is what happens in more affluent areas. Parents want for their children an even better, more varied, more interesting life than the one they have enjoyed. Of course they want them to have children of their own, but all in good time. So when their daughter or son is propositioned they will weigh the risk against university, travel, adventure, then value themselves enough to say: "Thanks, but no thanks."

The more vulnerable are those who see a dead-end job or long-term unemployment as their fate. For them, a baby can be an achievement; someone of their own to love and look after. Is starting sex education at five the way to counter that? Absolutely not. Girls should learn about how lucky they are to live in a society in which they command both their own fertility and equal employment rights. Even today, there are large swathes of the world where women know no such freedoms. Boys should understand a baby commits them to financial support for 18 years. There should be an emphasis on what else they could do with their lives. All need to be shown that parenthood is a lifetime commitment and very hard work, while being alerted to the many alternate avenues to success. There are plenty of entrepreneurs, actors, artists, politicians and sportsmen who scored low at school.

There is no poverty of opportunity in Scotland (ask any immigrant), but there is too much poverty of aspiration. That is the enemy. Address it, emphasise personal responsibility, encourage parents to open up with their children and, just as in the Netherlands, the problem of teenage pregnancy could be halved by 2010.