The calendar has long been medicine's shorthand for aging, but the body keeps a different kind of time. A study of nearly 5,000 middle-aged and older Chinese adults found that a four-marker biological age score — drawing on creatinine, glucose, and inflammation levels alongside chronological age — anticipated cognitive decline more faithfully than birth year alone. The finding invites a quiet but consequential question: if the body ages at its own pace, should medicine learn to listen more carefully to what it is actually saying?