While everyone grows older with each birthday, not everyone looks - or wants to look - his age.

Some youthful lookers are blessed with good genes, others with arguably good habits.

With modern aesthetic procedures, the vain can battle age spots, varicose veins and a sagging body.

As to why some stay youthful-looking longer, Dr Caroline Low, senior family physician in anti-ageing medicine at Raffles Hospital, said: 'It's 50 per cent nature and 50 per cent nurture.'

Her advice to those seeking the elixir of youth is to refrain from smoking, excessive alcohol or caffeine, or eating preserved or charred food.

Also, avoid getting stressed, get eight hours of sleep every night, reduce your calorie intake and exercise regularly.

'It boils down to the ABCs of lifestyle,' she said.

Other doctors MYB spoke to agreed, adding that while good genes give you a headstart, other factors that count include the rate at which your cells age, exposure to the sun and the amount of free radicals in your body.

Consultant plastic and cosmetic surgeon Ivor Lim from The Plastic & Hand Surgery said: 'Ageing is often cellular failure. The reason we get old is because our stem cells are no longer there to repair our bodies.'

Dr Andrew Khoo, consultant plastic surgeon at Aesthetic & Reconstructive Centre, said the single most important 'reversible' factor in the ageing of the skin is the amount of exposure to ultraviolet radiation when out in the sun.

Dr Patricia Yuen, consultant dermatologist at Pacific Healthcare Specialist Centre, said that everyone ages at a different rate and that is predetermined by genetics.

'However, sun damage and environmental pollutants accelerate the rate of natural ageing,' she said.

Free radicals can also age our bodies by damaging the genetic information held in cells, leading cells to die.

Although free radicals - oxygen molecules formed as a by-product when cells convert energy into a form they can use - are necessary, too much can harm and age the human body.

When a person ages, his body undergoes various changes, including hormonal ones, said Dr Julinda Lee, consultant obstetrician and gynaecologist with a specialist interest in age management at Pacific Healthcare Specialist Centre.

Such changes occur as a result of increasing cellular damage and a decline in the function of the various endocrine glands that produce hormones, she said.

Put simply, fewer hormones can affect the skin, hair, body composition and body shape.

Likening the ageing process to the 'wear and tear' in a car engine, Dr Leslie Leong, consultant orthopaedic surgeon at Island Orthopaedic Group, said: 'The body's ability to make repairs starts to slow down and its ability to regenerate new tissue declines with age.'

The result: collagen production and elastin in the skin decreases, causing the skin to sag as it loses its ability to retain moisture or retract after stretching.

Skin also gets coarser and drier and pores on the face may enlarge. Dead skin cells are shed less quickly and new skin cell growth is slower.

Asked why sagging was such a pronounced feature of ageing, Dr Low said: 'As we grow older, gravity prevails.'

A woman's breasts usually sag due to the pull of gravity and the breasts' increasing proportion of fat, which is softer and does not hold up as well. Young breasts have a higher proportion of glandular tissue.

While different areas begin ageing at different times, most doctors MYB spoke to agreed that the effects start showing in the late 30s or early 40s.

Asked why men seem to age better than women, Dr Low attributed it to the gradual decline in testosterone in men as opposed to the plunge into a state of zero female hormones during menopause.

Dr Derrick Aw, consultant dermatologist in the department of medicine at National University Hospital, added: 'Oil glands are generally more active and stay more active in men, which reduces the dry, sallow look. Women also tend to have slightly more facial fat than men, hence the sagging effect.'

Modern medical technology can mask the effects of ageing.

Dr Lim said: 'I'm seeing a lot of people in their 30s or 40s and more men who are ready to go for aesthetic treatments.'

Such expensive aesthetic procedures include plastic surgery, facelifts and Botox jabs.

There is always the healthy option too: a balanced diet filled with antioxidants, plenty of water and regular exercise.

Dr Low gave this tip: 'Exercise is the only thing that keeps people young. If you've exercised for more than 10 years continuously, you will look as young as the day you started regular exercise.'

This article was first published in Mind Your Body, The Straits Times on July 30, 2008.