In short
Biological age is an estimate of how much physiological wear a person's body shows relative to population norms, as opposed to chronological age, which is simply the number of years since birth.
Two people of the same chronological age can have measurably different physiological function, disease risk, and biomarker profiles — biological age is an attempt to quantify that difference using data such as blood biomarkers, epigenetic methylation patterns, physical performance tests, or a combination of these, typically calibrated against a reference population's chronological age or health outcomes.
There is no single, universally agreed method for calculating biological age. Approaches include epigenetic clocks (like GrimAge or PhenoAge, built from DNA methylation), clinical biomarker composites (like the Klemera-Doubal method or PhenoAge's original clinical-chemistry version, built from blood panels), and functional-performance composites incorporating measures like grip strength, VO2 max, or gait speed. Different methods applied to the same person frequently produce different numeric "ages," because they are measuring different biological signals.
Biological age estimates are useful as a way to track the direction of change in response to a lifestyle or treatment intervention over time in the same person, using the same test — that is a reasonable, evidence-consistent use. Treating a single biological-age number as a precise, validated prediction of remaining lifespan, or comparing biological-age scores from different commercial tests as if they were interchangeable, goes beyond what the current science supports.