Your liver might be holding up just fine, but your brain could be 20 years ahead of you. That’s the premise of a new study from Stanford scientists, who developed a blood test that can estimate how old your organs really are—and how that biological aging might predict how long you live.

Using blood samples from nearly 45,000 adults, researchers tracked protein levels linked to 11 major organs. Organs that showed more protein “leakage” than expected for a person’s age were flagged as aging faster. Others seemed oddly youthful. And while that may sound abstract, the implications were very real.

“More than 60% of people with 8+ extremely aged organs at blood draw died within 15 years,” the researchers wrote. Meanwhile, those with just two to four aging organs still faced more than double the risk of death compared to people with younger profiles.

Can a Blood Test Reveal How Long You Will Live?

The test doesn’t change your DNA—it reads between the lines. When organs break down, they leave behind biological clues in the bloodstream. Researchers trained AI to interpret those patterns and spot which organs were hanging in there, and which ones were speeding toward trouble.

Some of the biggest surprises were which organs mattered most. “I expected many more organs to be linked to longevity, but our data suggest the immune system and brain are key,” lead author Hamilton Oh, PhD, told StudyFinds. “These systems may be the guardians of our whole body.”

Having a young brain alone cuts death risk by 40%. A youthful immune system? 42%. Combine both, and you get the strongest odds of a longer life. By contrast, having youthful kidneys, lungs, or skin didn’t give the same advantage, and having youthful arteries was oddly linked to a higher death risk.

The good news: your organs’ fates aren’t sealed. The study found that factors like smoking, processed meat, and poor sleep aged multiple organs. Meanwhile, exercise, fish, and even cod liver oil helped some stay younger. A few over-the-counter supplements, including ibuprofen and vitamin C, were associated with slower aging in the brain, pancreas, and kidneys.

In short, aging isn’t a one-size-fits-all collapse. It’s more like a group project where some of your organs are coasting while others are doing all the work. And the slackers are bringing down your grade.