You're only 35, 38 or 45. So why are you suddenly more tired? Why doesn't your body bounce back like it used to? Why is maintaining your weight harder? Why does one bad night's sleep seem to affect your entire next day? Is this just what happens after 35—or is there more to the story? There may be. Your Birthday Is Only One Measure of Age Your chronological age is simple: the number of birthdays you've celebrated. Your biological age is different. It's an estimate of how your