Some of us are desperate to know the answer. Others may think of nothing worse.

But, for those wanting a glimpse into their future to see how old their body is, there's no need to look any further — companies are increasingly trying to solve the riddle for you. 

Biological age tests, which featured on the latest season of Keeping Up With the Kardashians, can, in theory, distinguish between your chronical and biological age. 

The former refers to how many years you've lived, while the latter is an estimate of the body's decline based on subtle markers on your DNA. 

But new research has suggested the they may not be as accurate as first thought in determining your true 'biological age'. 

US scientists who carried out the study warned their findings had 'important repercussions for the interpretation of commercial biological age tests'.  and will help to refine their fidelity and utility for the future'. 

Epigenetics is a well-established field, with dozens of companies producing their own saliva and blood 'clock' tests.

But questions have been raised over whether epigenetics themselves can be used to recommend lifestyles changes.

The traditional view of ageing is that it's caused by a slow build-up of damage at a cellular level. Just like a car, bits of us get broken or wear out.

It can be difficult to put a number on this. That's where an epigenetic clock test comes in.

Priced between £150 and £300 on average, they measure so-called DNA methylation levels.

 This is the extent to which special molecules, called methyl groups, have latched on to the DNA in your cells.

These have been described as being like barnacles attaching themselves to the hull of a ship and slowing it down.

In the study, researchers at Harvard University assessed how stem cells within a tissue or organ impacts its epigenetic clock. 

Tests on mice showed they differed by body part, with stem cells in skeletal muscle, trachea and the even blood proving to have a lower age than more mature cells.

By comparison, stem cells had a similar epigenetic age to mature cells in the skin and intestine. 

Writing in the journal, Stem Cell Reports, the researchers said that muscle and blood cells 'that undergo age-associated changes in abundance register a distinct epigenetic age from bulk tissues'. 

Further experiments also showed the epigenetic age of a stem cell was linked to the rate at which it divides and makes new cells.

They concluded the epigenetic age of a tissue is influenced by the activity of stem cells. 

But stem cells can change in response to ageing, injury or disease, they noted, meaning results of biological ageing tests may not be accurate. 

Their findings 'will help to refine' the tests 'fidelity and utility for the future, the researchers added.

The first epigenetic tests came after German geneticist Professor Steve Horvath developed an ageing 'clock' at the University of California, Los Angeles in 2011.

He measured specific epigenetic patterns linked to ageing.

This allowed him to produce a model that estimated someone's biological age based purely on methylation levels.

When testing hundreds of people blindly, the biological age figures matched up with the majority's chronological age — suggesting they were accurate.

This meant the clock served as a useful comparison to see how much younger or older your cells are than the average person of your age.

Since then, companies such as Zymo Research and Epimorphy and Chronomics have brought out tests.

On a recent episode of The Kardashians, Kim, mum Kris and sister Khloe, who all took tests, discovered Khloe's biological age was reportedly 28 — 11 years younger than her actual age, 39. 

Kim, 43, meanwhile, was given a biological age of 34, with Kris six years younger than her actual age.