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.
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