Ask your friends to guess your age and if they come up with numbers that are far higher than what your birth certificate says, you may be one of those who is more exposed to harmful substances in the environment.
Exposure to cigarette smoke and other toxins in the environment may make you older than your actual years, a study said.
"The rate of physiologic, or molecular, aging differs between individuals in part because of exposure to 'gerontogens' - environmental factors that affect aging," said Norman Sharpless from University of North Carolina in the US.
From a public health perspective, cigarette smoke is likely the most important gerontogen, Sharpless added.
Ultraviolet (UV) radiation from the sun makes us older too and researchers showed that chemotherapy treatment is also a strong gerontogen.
"By identifying and avoiding gerontogens, we will be able to influence ageing and life expectancy at a public health level," Sharpless noted.
In the future, blood tests evaluating biomarkers of molecular age might be used to understand differences amongst individuals in ageing rates.
The study appeared in the journal Trends in Molecular Medicine.