When we remark with surprise that someone “looks young” for his or her chronological age, we are observing that we all age biologically at different rates. Scientists have good evidence that this apparent difference is real. It is likely that age changes begin in different parts of the body at different times and that the rate of annual change varies among various cells, tissues, and organs, as well as from person to person. Unlike the passage of time, biological aging resists easy measurement. What we would like to have is one or a few measurable biological changes that mirror all other biological age changes without reference to the passage of time, so that we could say, for example, that someone who is chronologically eighty years old is biologically sixty years old. This kind of measurement would help explain why one eighty-year-old has so many more youthful qualities than does another eighty-year-old, who may be biologically eighty or even ninety years old.