A man's "biological clock" starts ticking seven years earlier than a woman's, says a study revealing a man's fertility declines significantly as he gets older.

A man's "biological clock" starts ticking seven years earlier than a woman's, says a study revealing a man's fertility declines significantly as he gets older.

A man's ability to father a child starts to decline when he is 24. A woman's fertility, on the other hand, begins to decrease from the age of about 30, research published today shows.

The older a man is, the longer it will take his partner to conceive, irrespective of her age. Women whose partners are at least five years older than them have less chance of conceiving within a year than women whose partners are the same age, or younger.

The research, which is published in the journal Human Reproduction, shows the chance of conceiving in six months decreases by 2 per cent for every year the man is older than 24.

The study is the first to quantify the decline in fertility of a man who has no known infertility problem. It shows that a man's age, as well as a woman's, is an important factor in conception. Previous research has shown a man's sperm starts to deteriorate in quality from the age of 18, but the latest findings indicate that there is no impact on his ability to produce children until six years on.

The conclusions were drawn by teams from Bristol University and Brunel University who analysed data provided by the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC), known as the "Children of the 90s" study.

ALSPAC, designed to evaluate the effects of personal, social and environmental factors on the development of children from early pregnancy onwards, looked at 85 per cent of families in Avon's health authority whose babies were born between 1 April 1991 and 31 December 1992. Of the couples who took part who had planned pregnancies, more than 8,500 stated the time it took to conceive. The researchers used this data to calculate the effect of a man's age on the time it takes for his partner to become pregnant.

Kate North, a senior researcher at Bristol University and a co-author of the study, said: "It is really difficult to quantify the effect of men's age on fecundity because it is confounded by so many factors.

"But after adjusting carefully all the variables we still found that women with older partners were significantly less likely to conceive in under six or 12 months. Because of the size and composition of the study we are confident that our findings are robust and the effect is real."

George Hogewind, a consultant gynaecologist at the Centre for Reproductive Medicine in London, said: "The study shows the body is designed for men and women to have children at 16 or 17 years old, but society cannot accept this and is pushing people to have children later and later."

The researchers concluded that, even in a couple who prove fertile, the likelihood of it taking more than a year to conceive is nearly double when he is older than 35 than it was when he was younger than 25, from about 8 per cent to about 15 per cent.

Dr Chris Ford, who works for the university division of obstetrics and gynaecology at St Michael's Hospital, Bristol, said: "It tells us that, to some degree, men as well as women have a biological clock that starts ticking as they get into their thirties.

"It also indicates that paternal age is another factor to be taken into account when doctors are looking at the prognosis for infertile couples."