"Seventy per cent of ageing can be prevented by good, healthy habits and the more you look after a part of your body the slower it will age," says Dr Michael Roizen, author of Real Age, Are You As Young As You Can Be? (Thorsons, pounds 9.99). "The bits that matter to us most will age slowest."
Someone concerned about their weight but not their skin will have a lower fitness than skin age - they'll work out but forget to use sun protection. Someone whose mother had skin cancer may well be the opposite.
But what about me? To put these theories to the test, I had the biological age of my body, skin and teeth checked - would they be older or younger than my chronological age of 30?
DOWN AT THE SKIN CLINIC
The state of affairs: I'm a sunbather and travel abroad at least three times a year. My sunscreen use is sparodic. My normal skin-care regime involves moisturiser when I feel old and haggard, and so far I've been lucky as I don't have many lines.
The expert's view: Ravi Ratnavel, consultant dermatologist at the Thames Valley Nuffield Hospital in Buckinghamshire, examined me with the Neutrogena skin camera, which uses UV light to look under the skin and pinpoint sun- damaged areas that don't yet show on the surface.
On the surface, he says, I have good skin for a 30-year-old, just slightly dehydrated. Underneath, however, is a different story. The camera reveals a mass of tiny brown spots, fine lines and a few white speckles - the bits I haven't destroyed by sunning myself. "I'd say your skin is about a quarter older than you are - closer to the skin of a 40-year-old than a 30-year-old - and you're very lucky that it hasn't started to show by now," he explains. The sun damage is concentrated mainly across my nose and forehead and the lines will probably become more obvious to the naked eye over the next few years - the freckles and spotting will appear as age spots. "You're lucky you don't smoke or the damage could be much worse," he says, revealing my skin age to be 38. "If you don't wear sun protection from now on you'll have the skin of a 60-year-old by the time you're 40 - not to mention your risk of skin cancer."
How to lower your skin age
Protect yourself from the sun: 80 per cent of skin damage occurs because of external factors, especially the sun. "Even in the UK we should protect our skin with SPF15 between May and October," says Dr Ratnavel. "When you go on holiday wear at least a factor 25 - and avoid the midday sun."
Don't smoke: Smoking destroys collagen and leads to sagging and wrinkling. The advice all our experts gave was to quit. "If you smoke a pack of cigarettes a day you can add eight years to every part of your body right now," says Dr Michael Roizen.
Moisturise daily: Dehydration makes collagen brittle and promotes wrinkling.
Use products with Retinol: The only cosmetic product conclusively shown to reverse sun damage is prescription-only Retin A. Cosmetic companies are now using a derivative of this, called retinol, in many anti-ageing products. It will help. Vitamin C-based products have also shown benefits.
AN AFTERNOON IN THE GYM
State of affairs: I've gained weight recently but I go to the gym four times a week and can run five miles easily. I don't smoke or drink coffee and my diet is OK.
The expert view: Peter King, fitness manager at The Gym at County Hall, tested my heart-rate, blood pressure, body fat, stamina and flexibility. "These are the most important age determinants - and they're relatively easy to alter."
I weigh 10 stone, which seems OK, but my 31 per cent body fat makes me technically obese. At 30, I should have just 24-28 per cent - 31 per cent puts me closer to age 50.
My resting heart rate is 93 beats a minute - a healthy, fit person should measure 60bpm. However, my blood pressure is on the low end of normal and I score an excellent on the fitness test. Peter thinks I have a naturally high heart rate.
I use a huge ball to measure my flexibility. "As we age, the muscles tighten," says Peter. I don't stretch after my workouts so I'm not surprised I only score an OK. Things aren't looking good, so I'm shocked when Peter pronounces my body age as 25 - on course to have a 48-year-old body age when I'm 70. "You scored well on the things that matter most - blood pressure, stamina and flexibility," he says.
How to lower your body age
Eat healthily: Those with lowest body age eat the most nutrients per calorie they take in. Eat 10 different fruits and vegetables a day and at least six other low fat/high fibre/moderate protein foods. Variety is the key.
Aim for a 60bpm heart rate: First thing in the morning, take your pulse - if it's more than 60, you need to stress less and exercise more. Experts recommend 15 minutes' relaxation time and at least 20-30 minutes of "activity" (eg walking) a day. Exercise and relaxation also lower blood pressure. High blood pressure increases body age by up to 20 years. Aim for 115/76.
Increase muscular tone and strength: Every decade after age 30 we lose 7 per cent of our muscle mass and replace it with fat. Prevent this through strength training work like weight-lifting.
Increase flexibility: stretching is important - the stiffer you are the older you look. To test, sit on the floor, legs straight out, heels together. Bending from the waist, reach as far past your feet as you can and place a coin on the floor. If you're under 35, reaching 8cm+ beyond your heels is excellent, not being able to reach your heels is poor. Over 35, 7cm+ past your heels is excellent.
A TRIP TO THE DENTIST
State of affairs: I can't say I like my teeth. When I go to the US they can tell I'm British because they're so crooked and a teenage accident with a bottle top has left a huge chip, front centre. Still, I go to the dentist every six months and haven't had a filling for three years.
The expert verdict: Dr Russell Craddock, a dental surgeon at Dentics, a cosmetic dentists, checks discoloration, shape, texture and gum health. "Bad dental habits can add years," he says.
He asks me to smile. Apparently, how many teeth I show is a clear giveaway to my tooth age. "The lips drop about a millimetre every 10 years - when you're young you show the top teeth but none of the lower." My smile shows little of my bottom teeth, placing me under 30.
Next, colour. Our teeth darken as we age - the yellower the teeth the older they're perceived to be (smokers often have a high tooth age). Yet again my smoke-free life scores me points - as does the fact that I don't grind my teeth, a common night-time response to stress. This causes discolouration as well as the wear reveals the dark dentine part of the tooth. "A 30- year-old who grinds could have the teeth of a 60-year-old," says Russell.
The more worn your teeth, the higher your tooth age. A young looking tooth will have a square-oval shape and smooth, shiny enamel surface. "You've got very little wear - in fact, they're the shape and texture I'd expect to see on an 18-year-old," says Russell. I'm thrilled. However, I lose marks because they are chipped and crooked. "The more symmetrical your teeth, the lower your tooth age," says Russell.
Finally, the state of our gums really defines our tooth age. As we get older our gums recede. This reveals the dark dentine and leads to tiny black triangles between the teeth. I've got none of these.
My tooth age is estimated at 24 - dropping to 18 if I got them cosmetically straightened.
How to lower your tooth age
Good oral hygiene: Flossing is a vital anti-ager - it keeps the gums healthy. You should also brush twice a day, but gently. "Smokers scrub to remove stains, which wears teeth," says Russell. Brush lightly and don't use abrasive toothpastes.
Avoid teeth stainers: No tea, coffee, red wine.
Stop grinding: A dentist can tell if you do this at night and can supply a protective mouthguard.
Sort out your symmetry: Cosmetic dentistry can straighten, lighten and lengthen teeth, dramatically reducing tooth age.