More than 100 Britons have now taken a revolutionary blood test to see how fast they are ageing, and which might be used in the future to indicate statistically how long they have got left to live.
The company behind the "lifespan" test believes ten times as many people in the UK will take the £650 blood check next year, and millions more worldwide will be tested by the end of the decade.
It also expects the test to be used as part of the standard medical check-up required by insurance companies, just as they now ask about family history of disease and whether someone is a smoker or obese.
However, some experts have warned that there is still not enough known about telomere testing to provide people with any meaningful medical advice, and one Nobel prize-winner has told The Independent that 99 per cent of people who take the test will not gain any benefit.
It is claimed the blood test estimates how fast someone is ageing by measuring the length of microscopic structures at the ends of each chromosome called telomeres, which keep each chromosome from falling apart when cells divide, much like the plastic tips at the ends of shoelaces stop them from fraying.
Telomeres shorten after each cell division and animal studies have shown that a high percentage of short telomeres in blood cells is associated with a shorter-than-normal life expectancy, which is why blood tests could provide a guide to ageing and life expectancy.
The Spanish makers of the test, Life Length, said that more than 1,000 people worldwide had volunteered to take the blood test since it became commercially available earlier this year.
"We consider that this will become as standard a medical diagnostic test as cholesterol testing is now," said Stephen Matlin, chief executive of Life Length, which is based in Madrid.
"If you look at cholesterol testing since the early 1980s, in a period of 15 years testing volume went from nothing to about 100 million a year," Mr Matlin said. "Today there are 500 million cholesterol tests a year. If we do one per cent of this, we are doing well. We hope to be testing millions of people by 2020," he said.
The company plans to lower the price of the test by 20 per cent a year for the next five years so that it costs no more than about £65 by 2017, bringing it within the price range of millions of new customers.
Professor Carol Greider of Johns Hopkins University in Baltimore, who won a Nobel prize in 2009 for her telomere work, warned however that only about 1 per cent of the population, those with the shortest telomeres, could benefit from the test at the moment – although she accepts things might change in the future.
"These people in the first percentile with the shortest telomeres are known to be at risk of certain diseases, such as pulmonary fibrosis or bone-marrow failure. So knowing your telomere length can save lives, but only for 1 per cent of the population. I'm not sure how the remaining 99 per cent will benefit," Professor Greider said.
Life Length has nevertheless been in discussions with the insurance industry about introducing the test as part of the standard medical check-up that customers usually take before being offered life or medical insurance.
Mr Matlin explained: "This will be a test that will give an incremental insight into health risks. It's not something new to insurance companies, they are already doing this. We want this test to become a standard biomarker."
Life Length began offering the test commercially in Britain last September. Doctors send their patients' blood samples to The Doctors Laboratory, the largest independent provider of medical diagnostics in the UK, which has an exclusive UK licence with Life Length.
"We don't work directly with the public in a normal setting, we work directly with physicians. It's the physicians who understand and interpret the results for their patients," Mr Matlin said.
Any wider uptake of the telomere test would be controversial because some experts have questioned the claims made about how accurate it is, and what it can actually predict in terms of ageing and longevity.
Mr Matlin said the telomere test was based on statistical associations between the average length of telomeres in a person's white blood cells, the percentage of short telomeres in each blood sample and the age of the individual.
Some people age faster than others and the aim of the test is to estimate someone's true biological age as opposed to their actual chronological age, he said.
"A 30-year-old would on average have longer telomeres and a lower percentage of short telomeres than a 60-year-old. But two 30-year-olds might have quite different numbers and the more that time goes by, the greater the divergence of results," Mr Matlin said.
"We have to be very careful. We are not saying here that we are making a prediction about the longevity of a specific individual," he said.
"However, it is has been well established by researchers at many different facilities that, statistically, living creatures with longer telomeres and a lower percentage of short telomeres have longer lifespans and healthier lifespans than the opposite," he added.
However, Professor Greider said she could not see how the Life Length test was able to determine the percentage of short telomeres with any accuracy and that suggestions of it being able to distinguish between chronological age and biological age were fanciful.
"Telomeres differ from person to person, even if they are the same age. If you sent me a sample of blood from someone I could only say whether they are between 17 and 70. I don't know how they plan to determine someone's biological age," Professor Greider said.
Mr Matlin said telomere tests could provide useful medical information, and pointed to the analogy of cholesterol testing, which is used to provide a statistical association between cholesterol levels and the risk of developing blocked arteries, heart disease and strokes.
"If you look at people with high cholesterol, then statistically they are at greater risk of cardiovascular disease, but that doesn't mean that someone couldn't live their entire life with high cholesterol without ever having a heart attack or cardiovascular disease." Mr Matlin said.
"And likewise people with low cholesterol could have heart attacks. That does happen, but at a statistical level people with high cholesterol are at greater risk than people with low cholesterol, and telomere length is strongly linked to longevity at a statistical level," he said.
One day it might even be possible to manipulate telomere lengths with drugs.
Mr Matlin said: "There is no pharmaceutical drug yet to regulate telomere length, but we are doing a lot of work with the pharmaceutical industry, and that will happen, but we don't know when it will happen."
Ageing through the ages: Humanity's obsession
The fear of growing old – and associated attempts to stay forever young – are probably as ancient as civilisation itself. Life Length's measurement of the ageing process using telomeres has echoes of the Thread of Life in Ancient Greek myth. The Greeks were also among the first to theorise concepts like the Fountain of Youth and other elixirs for immortality that appear in the legends of most cultures: from the Amrit of Hindu tradition to the Philosopher's Stone of European mythology. For the Greeks, the ultimate expression of youth and beauty was the goddess Aphrodite, but in more recent times literary characters such as Dorian Gray and Peter Pan stand as testament to mankind's enduring obsession with youth. The extraordinary leaps made in the science of DNA have led to fevered speculation that immortality might actually be within our grasp, with some theorising that telomeres – the tiny structures at the end of our DNA – hold the key.