Believe it or not, your lungs are six weeks old - and your taste buds just ten days! So how old is the rest of your body?
Many people fret about ageing, little realising that whatever your age in years, some of your body parts are just a few weeks, even days, old.
This is because they are constantly renewing themselves. Here, lets look at how old you REALLY are...
The liver is known for its amazing capacity to repair and re-grow itself thanks to its rich blood supply.
This means it can continue with its main job of flushing toxins out of the body.
If you've ever wondered why even heavy drinkers can sometimes improve the state of their liver, it's because liver cells only have a life span of around 150 days.
'I can take 70 per cent of a person's liver away in an operation and around 90 per cent of it will grow back within two months,' explains David Lloyd, consultant liver surgeon at Leicester Royal Infirmary.
However, in heavy drinkers the parenchymal cells - the liver's main cells - can become so damaged that scar tissue forms, a condition known as cirrhosis. So though a healthy liver can regenerate itself, with cirrhosis the damage is permanent - and sometimes fatal.
TASTE BUDS AGE: 10 DAYS
The tongue is covered with around 9,000 taste buds that help us to detect sweet, salty, bitter or sour flavours, explains Professor Damian Walmsley, scientific adviser to the British Dental Association.
The taste buds themselves are a collection of cells on the surface of the tongue, each housing about 50 taste cells. The buds renew themselves every ten days to two weeks.
However, anything that causes inflammation, such as infections or smoking, can damage the taste buds and affect their renewal - deadening their sensitivity.
BRAIN AGE: SAME AS YOUR AGE
Most of our cells that last a lifetime are found in the brain, explains John Wadley, consultant neurosurgeon at Barts and the London Hospital.
'We are born with all the brain cells we'll ever have - around 100 billion - and most of the brain does not regenerate as it gets older.
In fact, we actually lose cells, which is the underlying reason for dementia and why head injuries are so devastating.
'There are, however, two areas of the brain that do regenerate,' says Mr Wadley. 'The olfactory bulb that governs our sense of smell, and the hippocampus, which is an area for learning.'
HEART AGE: 20 YEARS
Until recently it was thought the heart couldn't renew itself. However, a study at New York Medical College found it is actually dotted with stem cells that constantly rejuvenate it - at least three or four times over a lifetime, say the researchers.
LUNGS AGE: 2-3 WEEKS
The cells in the lungs constantly renew themselves, explains Dr Keith Prowse, vice-president of the British Lung Foundation.
However, the lungs contain different cells that renew at different rates. The alveoli or air sac cells - needed for the exchange of oxygen and gases - deep in the lungs have a steady progress of regeneration that takes about a year.
Meanwhile, the cells on the lung's surface have to renew every two or three weeks.
'These are the lungs' first line of defence, so have to be able to renew quickly,' says Dr Prowse.
The lung disease emphysema can prevent this regeneration as it begins with the destruction of the alveoli, which creates permanent 'holes' in the walls of the lungs.
EYES AGE: SAME AS YOUR AGE
Your eyes are one of the few body parts that don't really change during your life.
The only part that is constantly being renewed is the cornea, the transparent top layer. If this is damaged, it can recover in as little as 24 hours, says Dr Rob Hogan, president of the College of Optometrists.
'The cornea has to have a smooth surface, so you can focus properly. That's why the cells renew themselves so quickly.'
Unfortunately, this isn't the case with the rest of the eye - as we age, the lens loses flexibility, which is why we struggle to focus as we get older.
SKIN AGE: 2-4 WEEKS
The epidermis or surface layer of the skin is renewed every two to four weeks.
This rapid turnover occurs because skin is the body's outer protection and is exposed to injury as well as pollution. Despite this constant renewal, we still get wrinkles as we get older. That's because the skin loses collagen - and elasticity - with age.
BONES AGED: 10 YEARS
The skeleton is constantly replacing itself, explains Dr Peter Selby, an osteoporosis expert based at Manchester Royal Infirmary.
It takes around ten years to do this completely.
Old bone is broken down by cells called osteoclasts and replaced by bone-building cells known as osteoblasts.
At any one time we have a mixture of old and new bone as the turnover rates differ throughout the body.
But when we hit middle age this renewal process slows down, so our bones tend to get thinner, which is why osteoporosis sets in.
INTESTINES AGE: 2-3 DAYS
OUR intestines are lined with cells known as villi - these are tiny, finger-like branches that increase the surface area and help the intestine to absorb nutrients.
They have a very high turnover rate and can be replaced every two to three days, explains Tom MacDonald, professor of immunology at Barts and the London Medical School.
This is because they are constantly exposed to chemicals such as highly corrosive stomach acid that breaks down food, so they're constantly under attack.
The rest of the intestine protects itself with a layer of mucus, although this barrier cannot withstand the stomach acid for long - so the cells here renew themselves every three to five days.
NAILS AGE: 6-10 MONTHS
Our nails are made of cells rich in a tough protein called keratin. Fingernails grow by 3.4mm every month - almost twice as fast as toenails.
Meanwhile, it takes ten months for a full toenail to grow, but only six months for a fingernail. This may be because they have a better blood supply and therefore better circulation.
The nails of younger people and men grow faster, which may also be because they have better circulation.
Bizarrely, the little finger nail grows much more slowly than other fingernails, although it's not clear why.
Generally speaking, the growth rate of nails also depends on age and conditions such as psoriasis, which can affect the tissue from which the nail grows.
RED BLOOD CELLS AGE: FOUR MONTHS
These are the body's vital transport system, carrying oxygen to every living tissue and carrying waste away.
They wear out every four months, after which the liver removes any remaining iron that is needed for healthy red blood cells before the remaining cell is destroyed in the spleen.
Since they can be also lost through injury and menstruation, the body constantly makes more.
HAIR AGE: 3-6 YEARS
The age of your hair depends on how long it is, but it generally grows 1cm each month, explains hair restoration surgeon Dr Bessam Farjo.
Each individual hair lasts up to six years in women and three years in men. Eyebrows and eyelashes are renewed every six to eight weeks, but repeatedly plucking brows stops them growing because it disrupts this cycle.