Holding Back the Years ; Keira Knightley, 23 AGE-OLD PROBLEM: Changes to Your Diet and Exercise Routine Can Have a Major Influence on How You Age Jude Law, 36 Daniel Craig, 40 Julie Walters, 58 Helen Mirren, 63

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Byline: Angela Epstein

We all have to accept growing old as a fact of life. But even though changes to our body often happen imperceptibly, there are things we can do to head off the ageing process and maintain our good health. Here the experts reveal the changes you can expect in your body - and show ways to keep it in peak condition.

20s

WHAT STARTS TO GO

We may feel at the peak of physical fitness but signs of ageing are beginning. According to GP Dr Sarah Brewer, a change in growth hormones and hormone secretions can cause cholesterol to rise - though the rate of this will be compounded by lack of exercise, poor diet and stress.

As arteries begin to lose a little of their elasticity, blood pressure may start to rise slightly. We also start to lose some of our 100 billion brain cells.

Wrinkles and fine lines begin to appear. According to Dr Andrew Wright, a consultant dermatologist with Bradford NHS Trust, this is because production of collagen - the protein that acts as scaffolding to the skin - slows, and elastin - the substance that enables skin to snap back into place - has less spring.

Oil-producing sebaceous glands and the water-producing sweat glands become less active and turnover of new skin cells may decrease.

The amount of bone in the skeleton has reached its maximum when we are about 25 years old - this is known as the peak bone mass. Peak bone mass varies widely but is generally higher in men than in women.

HOW TO HOLD BACK TIME

Dr Brewer advises 30 minutes of daily exercise is the best pre-emptive strike against rising cholesterol, blood pressure and for maintaining glucose control (vital for avoiding type 2 diabetes).

As bone loss starts, ensure there is enough calcium and Vitamin D in the diet, advises Dr Peter Selby, an osteoporosis specialist based at the Manchester Royal Infirmary. The recommended daily intake for calcium in adults is 700mg. Good sources include dairy foods and leafy green vegetables. As Vitamin D is manufactured by sunlight on the skin, try to get 20 minutes' daily sun exposure. Eat Vitamin C-rich food such as peppers, oranges and strawberries as the vitamin plays an important part in maintaining collagen.

Dr Rob Hicks, GP and author of Control Your Blood Pressure, suggests having blood pressure tested every three to five years.

30s

WHAT STARTS TO GO

There was a balance between the build-up and breakdown of muscle in the body, but from now on breakdown is greater than build-up.

Though doctors are unsure why, one theory, at least for men, links this to a decrease in production of the hormone testosterone. From 35, the average woman's chances of conception are 50 per cent less than at 25 as the number of functioning follicles that contain eggs in the ovaries drops.

Male fertility also starts to decline - possibly due to a gradual build-up of genetic defects in sperm.

Male-pattern baldness begins in the 30s, triggered by changing levels of testosterone that cause the hair follicles to shrink. Grey hair starts to appear as colour-producing cells in the hair follicles - known as melanocytes - become less active.

Less saliva is produced to wash away bacteria so teeth and gums are more vulnerable to decay, says Damien Walmsley, professor of restorative dentistry at Birmingham University. Teeth may start to darken due to cumulative staining. Tiny cracks can also appear on the surface.

Lung function declines as the lungs begin to lose their elasticity, and the muscles in the joints of the chest become less efficient, says Dr Keith Prowse, chairman of the British Lung Foundation. It becomes more difficult to lose weight as our metabolism - the rate at which the body gets its energy from food - begins to slow down. Any excess calories we eat are more readily stored as fat.

HOW TO HOLD BACK TIME

To maintain lean body or muscle mass make sure you exercise the muscles above the waist as well as those below, advises Professor Adam Carey, head of nutrition and lifestyle for the Welsh Rugby Football Union. …