There's no denying the ticking of a woman's biological clock - but men are not immune, either. French doctors have found that the quality of sperm starts to deteriorate by 35, so that by the time a man is 45 a third of pregnancies end in miscarriage. Here, with the help of leading clinicians, Angela Epstein identifies the ages when different parts of the body start to lose their battle with time. • BRAIN Starts ageing at 20 As we get older, the number of nerve cells - or neurons - in the brain decrease. We start with around 100 billion, but in our 20s this number starts to decline.

By 40, we could be losing up to 10,000 per day, affecting memory, co-ordination and brain function.

In fact, while the neurons are important, it's actually the deterioration of the gaps between the brain cells that has the biggest impact, says Dr Wojtek Rakowicz, a consultant neurologist at Imperial College Healthcare NHS Trust in London .

We all assume grey hair and wrinkles are the first signs of ageing, but some parts of your body are worn out long before you look old.

These tiny gaps between the end of one brain nerve cell and another are called synapses. Their job is to ensure the flow of information from one cell to another, and as we age we make fewer. • GUT Starts ageing at 55 A healthy gut has a good balance between harmful and 'friendly' bacteria.

But levels of friendly bacteria in the gut drop significantly after 55, particularly in the large intestine, says Tom MacDonald, professor of immunology at Barts And The London medical school. As a result, we suffer from poor digestion and an increased risk of gut disease.

Constipation is more likely as we age, as the flow of digestive juices from the stomach, liver, pancreas and small intestine slows down.      

• BREASTS start ageing at 35 BY their mid-30s, women's breasts start losing tissue and fat, reducing size and fullness.

Sagging starts properly at 40 and the areola (the area surrounding the nipple) can shrink considerably.

Although breast cancer risk increases with age, it's not related to physical changes in the breast.

More likely, says Gareth Evans, breast cancer specialist at St Mary's Hospital, Manchester , our cells become damaged with age - as a result, the genes which control cell growth can mutate, causing cancer. • BLADDER Starts ageing at 65 Loss of bladder control is more likely when you hit 65. The bladder starts to contract suddenly, even when it's not full.

Women are more vulnerable to bladder problems as, after the menopause, declining oestrogen levels make tissues in the urethra - the tube through which urine passes - thinner and weaker, reducing bladder support.

Bladder capacity in an older adult generally is about half that of a younger person - about two cups in a 30-year-old and one cup in a 70-year-old.

This causes more frequent trips to the loo, particularly as poor muscle tone means the bladder may not fully empty. This in turn can lead to urinary tract infections.

• LUNGS Start ageing at 20 Lung capacity slowly starts to decrease from the age of 20.

By the age of 40, some people are already experiencing breathlessness. This is partly because the muscles and the rib cage which control breathing stiffen up.

It's then harder to work the lungs and also means some air remains in the lungs after breathing out - causing breathlessness.

Aged 30, the average man can inhale two pints of air in one breath. By 70, it's down to one. • VOICE Starts ageing at 65 Our voices become quieter and hoarser with age. The soft tissues in the voice box (larynx) weaken, affecting the pitch, loudness and quality of the voice.

A woman's voice may become huskier and lower in pitch, whereas a man's might become thinner and higher. • EYES start ageing at 40 Glasses are the norm for many over - 40s as failing eyesight kicks in - usually long-sightedness, affecting our ability to see objects up close.

As we age, the eye's ability to focus deteriorates because the eyes' muscles become weaker, says Andrew Lotery, professor of ophthalmology at the University of Southampton . • HEART Starts ageing at 40 The heart pumps blood less effectively around the body as we get older.

This is because blood vessels become less elastic, while arteries can harden or become blocked because of fatty deposits forming on the coronary arteries - caused by eating too much saturated fat.

The blood supply to the heart is then reduced, resulting in painful angina. Men over 45 and women over 55 are at greater risk of a heart attack.

A recent study by Lloyds Pharmacy found the average person in the UK has a 'heart age' five years older than their chronological age, probably due to obesity and lack of exercise. • LIVER Starts ageing at 70 This is the only organ in the body which seems to defy the aging process.

'Its cells have an extraordinary capacity to regenerate,' explain David Lloyd, a consultant liver surgeon at Leicester Royal Infirmary.

He says he can remove half a liver during surgery and it will grow to the size of a complete liver within three months.

If a donor doesn't drink, use drug or suffer from infection, then it is possible to transplant a 70-year-old liver into a 20-year-old. • KIDNEYS Starts ageing at 50 With kidneys, the number of filtering units (nephrons) that remove waste from the bloodstream starts to reduce in middle age.

One effect of this is their inability to turn off urine production at night, causing frequent trips to the bathroom.

The kidneys of a 75-year-old person will filter only half the amount of blood that a 30-year-old's will. • PROSTATE Starts ageing at 50 The prostate often becomes enlarged with age, leading to problems such as increased need to urinate, says Professor Roger Kirby, director of the Prostate Centre in London . This is known as benign prostatic hyperplasia and affects half of men over 50, but rarely those under 40. It occurs when the prostate absorbs large amounts of the male sex hormone testosterone, which increases the growth of cells in the prostate.

A normal prostate is the size of a walnut, but the condition can increase this to the size of a tangerine. • BONES Start ageing at 35 'Throughout our life, old bone is broken down by cells called osteoclasts and replaced by bone-building cells called osteoblasts - a process called bone turnover,' explains Robert Moots, professor of rheumatology at Aintree University Hospital in Liverpool .

Children's bone growth is rapid - the skeleton takes just two years to renew itself completely. In adults, this can take ten years.

Until our mid-20s, bone density is still increasing. But at 35 bone loss begins as part of the natural ageing process.

This becomes more rapid in post-menopausal women and can cause the bone-thinning condition osteoporosis.

The shrinking in size and density of bones can lead to loss of height. Bones in the back shrivel up or crumble between the vertebrae. We lose two inches in height by the time we're 80. • TEETH Start ageing at 40 As we age, we produce less saliva, which washes away bacteria, so teeth and gums are more vulnerable to decay.

Receding gums - when tissue is lost from gums around the teeth - is common in adults over 40. • MUSCLES Start ageing at 30 Muscle is constantly being built up and broken down, a process which is well balanced in young adults.

However, by the time we're 30, breakdown is greater than buildup, explains Professor Robert Moots.

Once adults reach 40, they start to lose between 0.5 and 2 per cent of their muscle each year. Regular exercise can help prevent this.. • HEARING Starts ageing mid-50s More than half of people over 60 lose hearing because of their age, according to the Royal National Institute for the Deaf.

The condition, known as presbycusis, happens due to a loss of 'hair cells' - tiny sensory cells in the inner ear which pick up sound vibrations and send them to the brain. • SKIN Starts ageing mid-20s The skin starts to age naturally in your mid-20s.

According to Dr Andrew Wright, a consultant dermatologist with Bradford NHS Trust, as we get older production of collagen - the protein which acts as scaffolding to the skin - slows, and elastin, the substance that enables skin to snap back into place, has less spring and can even break.

Dead skin cells don't shed as quickly and turnover of new skin cells may decrease slightly. This causes fine wrinkles and thin, transparent skin - even if the first signs may not appear until our mid-30s (unless accelerated by smoking or sun damage). • TASTE AND SMELL Start ageing at 60 We start out in life with about 10,000 taste buds scattered on the tongue. This number can halve later in life.

After we turn 60, taste and smell gradually decline, partly as a result of the normal ageing process.

This can be accelerated by problems such as polyps in the nasal or sinus cavities. It can also be the cumulative effect of years of smoking.

 • FERTILITY starts ageing at 35 Female fertility begins to decline after 35, as the number and quality of eggs in the ovaries start to fall.

The lining of the womb may become thinner, making it less likely for a fertilised egg to take, and also creating an environment hostile to sperm. Male fertility also starts to drop around this age. Men who wait until their 40s before starting a family have a greater chance of their partner having a miscarriage, because of the poorer quality of their sperm.

• HAIR Starts ageing at 30 Male hair loss usually begins in the 30s. Hair is made in tiny pouches just under the skin's surface, known as follices.

A hair normally grows from each follicle for about three years, is then shed, and a new hair grows.

However, with male-pattern baldness, changes in levels of testosterone from their early-30s affect this cycle, causing the hair follicles to shrink.

Each new hair is thinner than the previous one.. Eventually, all that remains is a much smaller hair follicle and a thin stump of hair that does not grow out to the skin surface.

Most people will have some grey hair by the age of 35. When we are young, our hair is coloured by the pigments produced by cells in the hair follicle known as melanocytes.

As we grow older, melanocytes become less active, so less pigment is produced, the colour fades, and grey hairs grow instead.