If you think that men can safely ignore their biological clock, think again. New research shows that men's fertility is just as susceptible to the effects of time as women's. The quality of sperm begins to deteriorate in the mid-thirties, according to a new French study. And by the time a man is 45, one in three pregnancies ends in miscarriage, regardless of the age of the mother.

The report has left experts arguing about how far modern lifestyles are to blame for failing sperm. And it's not just the ability to have children that may or may not be affected by environmental factors. There's plenty of anecdotal evidence of men siring children long after they've got their free bus passes. The French research suggests that older dads, from Picasso to Rod Stewart, are either the exception to the rule; or, "more likely, it signifies a recent reduction in male fertility due to men's increasingly unhealthy lifestyle", says the anti-ageing expert Dr Paul Clayton, research director of www.nutrition-matters.co.uk.

For women, fertility begins to decline from 35, and stops at 51, the average age of menopause in the UK. Since 2004, the majority of women have become mothers for the first time in their thirties – though births at this age have always been the norm, according to the Office for National Statistics. The current fertility rate for women in their thirties – 49 live births per 1,000 women – was last seen in this age group in the 1960s. The difference today is that most women have their first or second baby in their thirties, whereas in the 1960s, they already had at least two children.

The immune system is established soon after birth, and breastfed babies have stronger immune systems and experience fewer infections. In early and mid-life, the immune function is very effective at fighting infections caused by bacteria, viruses and fungus. By the fifties, that function can decline, opening the door to infections such as flu and diseases such as cancer and Alzheimer's, and heart disease. Our high risk of these diseases is relatively recent, claims Dr Clayton: "The Victorians, who were more physically active and had a healthier diet, had much lower risks of these killer diseases, largely because their immune systems remained strong well into old age."

Brain

It used to be thought that the brain was mature at around the age of 12, but findings from brain imaging now suggest that it's not fully mature until the age of 25. We are born with around 100 billion neurons or brain cells, each of which has multiple antenna-like dendrites that can potentially link up with tens of thousands of other cells.

By the age of five, our basic "cerebral architecture" is complete. New networks are most easily created up to the early teens – the optimum time for learning languages, maths, music, writing and co-ordinated movement, including sports and dancing. But the potential exists for something like a hundred trillion connections.

We start to lose brain cells in our twenties. It is now known that healthy adults can continue to build their brain and expand their mind at any age; the ability to grow new brain cells and develop new networks continues into old age. Severe mental decline is caused by disease. More commonly, any age-related loss of memory or motor skills is generally the result of increased inactivity and a lack of mental stimulation.

Senses

Sight: Difficulty focusing close up starts in the forties. The pupil slowly shrinks, restricting the amount of light that is able to get through. At an early stage, people manage presbyopia (old sight) by simply letting in more light. But most people need reading glasses by their mid-forties. Eye conditions including glaucoma, age-related macular degeneration, and cataracts become common after 60 – the age at which you're entitled to a free eye examination.

Hearing: Four in 10 people in their sixties, and six in 10 people in their seventies have age-related hearing loss, which is largely unavoidable as it's down to their genes. But the "MP3 generation" – 16- to 30-year- olds – is already experiencing prolonged buzzing in the ears, a sure sign of premature hearing loss. One in five of this age group regularly listens to an MP3 player for more than 20 hours a week, at noise levels above that of a pneumatic drill.

Taste: Taste buds first appear when the foetus is seven or eight weeks old, and by the third trimester, it can detect sweetness. Newborns have 10,000 taste buds on the tip of the tongue, the palate and in the throat. These gradually decrease with age, together with a continuing reduction in the amount and stickiness of saliva. By the age of 60, bitter tastes are less intense, though sweetness is not affected.

Smell: The ability to smell reaches a peak at the age of eight and then plateaus. Unlike taste, however, our ability to smell can remain robust. Recent research shows that 80-year-olds with good mental and physical health have the same olfactory powers as young adults. Women consistently outperform men in all tests of smelling ability.

Skin

The ageing process in our largest organ begins in the early twenties, but the signs of ageing are largely dependent on our genes, according to specialist nurse practitioner Marie Duckett ( www.fionamarie.co.uk). "Black and Asian skin ages best, while for someone with a fair complexion, facial lines can start to show by the mid-twenties," she says. But today's teens and younger children may well bear the ravages of time far better, she believes. "As children, the baby-boomer generation was sent out in the blazing sun in their swimming costumes and then smothered in calamine lotion at night once they'd turned lobster-red and their skin started peeling.

"Mothers today are far more careful and their children's skin should weather well, provided they avoid smoking and heavy drinking, and wear sunscreen and a sun hat," she says.

Sex drive

Men's sexual peak is at 22 – at least in terms of testosterone. After that, levels of the male hormone fall by around one per cent a year, with the amount of bio-available testosterone halving between the ages of 25 and 75. "But peak hormones don't necessarily mean peak sexual performance," says the psychotherapist Dr Michael Perring. Peak sexual performance, he says, is obviously closely associated with environmental factors, including availability of partners. "That's why men today are most active sexually from their late twenties, and still want to enjoy flings well into their thirties, when women their age are looking for a permanent relationship."

Women are said to be best in bed in their mid-thirties. But late motherhood has pretty well put a stop to that, says Dr Perring. "They're simply too tired, always assuming they don't have a baby in bed between them and their partner." He says that women's peak sexual activity now occurs in their forties. "It's not really about hormones. It's whether you've got the time, inclination and opportunity to focus on sex."

Fertility

If you think that men can safely ignore their biological clock, think again. New research shows that men's fertility is just as susceptible to the effects of time as women's. The quality of sperm begins to deteriorate in a man's mid-thirties, according to a new French study. And by the time he's 45, one in three pregnancies ends in miscarriage, regardless of the age of his partner, the researchers have found. The report has left experts arguing about how far modern lifestyles are to blame for failing sperm. And it's not just the ability to have children that may or may not be affected by environmental factors.

There's plenty of anecdotal evidence of men siring children long after they've got their free bus passes. The French research suggests that older dads, from Picasso to Rod Stewart, are either the exception to the rule – or, "more likely, it signifies a recent reduction in male fertility due to men's increasingly unhealthy lifestyle", says the anti-ageing expert Dr Paul Clayton, research director of Nutrition Matters (www.nutrition-matters.co.uk).

For women, fertility begins to decline from 35, and stops abruptly at 51, the average age of the menopause in the UK. Since 2004, the majority of women have become mothers for the first time in their thirties – though births at this age have always been the norm, according to the Office for National Statistics. The current fertility rate for women in their thirties – 49 live births per 1,000 women – was last seen in this age group in the 1960s. The difference, however, is that today, most women are having their first or second baby in their thirties, whereas in the 1960s, "a greater proportion of births were to women who already had at least two children", an ONS spokesperson explains.

Joints

Normal wear and tear is widely believed to lead to progressive osteoarthritis (OA) from the forties onwards. "Most people experience the first symptoms between the age of 40 and 50, usually in the knees, shoulders or hands, and assume that it's downhill all the way," says Phillip Conaghan, Professor of Musculoskeletal Medicine at Leeds University, and a council member of the Arthritis Research Campaign. "In fact, OA is rarely an inevitably degenerative disease, and the worst response to painful joints is to rest them."

Muscle loss naturally begins around 30 – and without regular exercise, muscle mass declines by 22 per cent between 30 and 70. "It's the muscles that support the joints, and intervention is too late once you're housebound," says Professor Conaghan. "It's best to view painful joints as a sign that you need further interventions: starting or increasing muscle-strengthening aerobic exercise, wearing shock-absorbing footwear, and keeping to a healthy weight."

High-impact repetitive sports or an injury to a knee or other joint in early adulthood can lead to inevitable OA in middle age, however. Improvements in hip and knee implants, and higher expectation of mobility have brought down the average age of the first hip or knee replacement – with implants now routinely offered to the under-sixties.