Katy Perry Says Her Biological Age Is 33. But Can a Test Really Tell How Old Your Body Is?

Biological age tests have jumped from research labs into direct-to-consumer kits and celebrity Instagram feeds. Katy Perry has told followers her biological age is 33. Here is what the science actually supports, and what it does not.

What is biological age and how is it different from chronological age?

Biological age estimates how quickly your body is aging relative to peers born the same year, while chronological age just counts calendar time. A test that puts a 60-year-old at 55 is saying that person’s body resembles the average 55-year-old’s.

The underlying concept is not fringe. “Biological age is real. There’s a difference between your chronological age and your biology, that’s for sure, and we can measure it by various means,” David Furman, a professor at the Buck Institute for Research on Aging, told Forbes.

How do biological age tests actually work?

Most aging clocks read DNA methylation, a chemical process in which methyl groups attach to DNA over time and alter the cytosine base, one of the four letters of DNA. The distribution of those tags is used to estimate biological age.

Steve Horvath pioneered the approach and developed the GrimAge test. “You can use methylation to measure time in all cells that contain DNA,” he told NPR. A higher proportion of methylated DNA at certain sites points to accelerated biological age.

Horvath spent more than 10 years as a UCLA professor working on the biomarker. “We spent over 10 years trying to understand what factors accelerate your epigenetic clock,” he told NPR. Smoking, poor diet, inactivity and “pretty much any lifestyle factor that is bad for you” push the clock forward, his research found.

What do biological age test results tell you?

You get a number, not a lifespan. Different clocks measure different biology, so each captures one slice of aging.

• GrimAge, which reads DNA methylation in blood and predicts higher risk of cognitive decline. It is named after the Grim Reaper.
• iAge, which tracks chronic inflammation and predicts greater vulnerability to heart disease and frailty.
• GlycanAge, which measures biomarkers reflecting immune function.
• Microbiome clocks, which estimate aging of the gut.

Luigi Ferrucci, a geriatrician and scientific director at the National Institute on Aging, urged restraint. “At this point, if you want to do it, it must be based on curiosity,” he told NPR.

How accurate are biological age tests?

Results can swing considerably, and no test predicts how long you will live. Estimates shift within a day, and physical activity can move them, so researchers suggest retaking a test at the same time of day for a fair comparison.

A 2025 study found professional German soccer players tested right after a match looked biologically far younger, possibly from physiological changes tied to intense exertion. Their numbers returned to baseline a few hours later.

The underlying process cannot be halted. “Aging is something that happens to all of us. Even if we optimize everything, it’s [only] the rate of change which is modifiable,” Bradley Elliott, a researcher at the University of Westminster and trustee of the British Society for Research on Ageing, told Forbes.