That said, the perfect compromise would be to try to complete your family before both partners reach the age of 35.
The TimesMarch 23, 2009
Do older men have less intelligent children?
New research suggests the older your father is when you are born, the lower your IQ, so how late is too late?Dr Mark Porter
New research suggesting that the age of your father influences your IQ - the older he was when you were born, the worse you are likely to fare when tested - is the latest addition to growing evidence that it is not just maternal age that matters when it comes to starting a family. The longer a couple delay, the poorer the outcome for all concerned. But how late is too late?
Back in the early Nineties, about a quarter of children were fathered by men over the age of 35; today it is closer to half, at least for married couples, who still account for the majority (just) of children born in the UK, with the average first-time mother and father now being aged 30 and 32 respectively.
There are lots of reasons why couples are waiting longer, but career and financial pressures feature highly - a trend likely to have been exacerbated by the current economic climate. But while there are obvious benefits to having more mature parents, these have to be offset against the medical implications - and there are many.
For women the main hurdle is declining fertility. While it is technically possible to conceive naturally right up until your last period, female fertility wanes dramatically after the age of 35, and by the time most women reach their late forties they are technically infertile.
Biological clock strikes for men too - at 35
Children of older fathers at risk of autism
The problems of being an older mum
A healthy couple in their twenties have a 25 to 30 per cent chance of conceiving each month. This falls to between 10 and 25 per cent when the woman is in her mid-thirties, and has plummeted to less than 5 per cent by the time she is in her early forties. What's more, nearly half of those who do manage to conceive at this age will miscarry within the first three months.
Much of this decline is due to genetic damage inflicted on a woman's eggs by a combination of environmental factors, such as toxins in the diet and natural background radiation. Women are born with a finite supply of eggs, and a 40-year-old egg is harder to fertilise and nurture than a 20-year-old one.
Tradition has it that advancing years do not have such a detrimental effect on men, who, unlike women, manufacture fresh sperm throughout their lives. But they are not actually manufactured from scratch, and the basic template that matures into a fully grown sperm is, like a woman's eggs, as old as the man - so male fertility wanes, too.
The fall starts to become significant when a man reaches his early forties, meaning that it takes longer to conceive, irrespective of the age of the would-be mother. When they are successful, the woman is also more likely to miscarry if her partner is over 40.
The effect of age on the risk of congenital abnormalities appears to be shared between the parents, too. The best known example in women is the link with Down's syndrome. If a woman is in her late twenties, the risk of her child having Down's is about 1 in 1,000. By the age of 35 it increases to 1 in 270, and by 40 it is closer to 1 in 100.
But less marked genetic mutations are thought to be a problem in older fathers as well, and the implications can be just as serious. Children born to men aged over 35 are more likely to have a cleft lip or palate, congenital heart defects, and to develop some forms of cancer, including leukaemia (a 50 per cent increase) and brain tumours (25 per cent increase).
There is also evidence of a link between paternal age and the chances of a child going on to develop autism, dyslexia or schizophrenia - the link with the last of these being particularly strong. Experts estimate that the trend towards delayed fatherhood could account for as many as 10 per cent of new cases of schizophrenia diagnosed each year.
But let's not be overly pessimistic. Torture statistics enough and they will tell you anything. In fact, the vast majority of older mums and dads will have trouble-free pregnancies and perfectly healthy children. Leaving it later may increase the risk of a range of complications - but a 50 per cent increase on a tiny risk is still only a tiny risk. And the medical implications need to be offset against the social and emotional benefits of bringing up a child in a more stable environment.
That said, the perfect compromise would be to try to complete your family before both partners reach the age of 35. It's a feat that my parents managed with ease - they were just 20 and 21 when they had me, which, statistically, means that I will probably live for ever.