Scientists believe they can do a blood test to check how fast a person's internal organs are aging and even predict which ones may soon develop problems.
The Stanford University team says they can monitor 11 major parts of the body, including the heart, brain and lungs.
They tested this on thousands of adults, mostly middle-aged or older.
1 in 5 fairly healthy adults over the age of 50 may have at least one organ that ages quickly, the results suggest.
And 1 to 2 in every 100 people may have some organs that are older than the person's age.
"While the idea of ??control can be scary, it can be an opportunity to intervene and avoid problems," the researchers say.
Knowing which organs are in rapid decline can help detect potential health problems, researchers say in the journal Nature.
For example, an "old-for-its-time" heart increases the risk of heart attack, while a rapidly aging brain may be more prone to dementia.
In the study, rapid aging of one or more organs was associated with a higher risk of certain diseases and death over the next 15 years.
The body parts they checked included:
Brain, heart, liver, lungs, intestines, kidneys, fat, blood vessels (arteries), immune tissue, muscles, pancreas.
One of the researchers, Dr Tony Wyss-Coray, explained: "When we compared the biological age of each of these organs for each individual with their counterparts in a large group of people without apparent serious disease, we found that 18.4% of those aged 50 years or older had at least one organ that aged significantly faster than average, and we found that these individuals are at increased risk for disease in that particular organ in the next 15 years."
Now the university has submitted documentation to retain copyright for the test, in case it can be used and sold in the future.
Professor James Timmons, an expert in age-related health and disease at Queen Mary University of London, has also studied biological age.
He said the latest findings were impressive, but there was a need to study more people, especially younger people from different ethnic backgrounds.