You may think you know how old you are, but your body doesn’t follow a calendar. That is, your chronological age, which measures how many years have passed since you were born, may not match your biological age , which reflects wear and tear on your body at a cellular level. Based on factors including genetics , lifestyle habits and medical history, you may be biologically older or younger than your chronological age. That’s because these two measures don’t always progress at the same pace,