A man's testosterone doesn't necessarily drop with age – it may fall due to poor health. Australian researchers measured the blood testosterone levels of close to 400 men over age 40 and found that levels didn't fall in those who were self-reportedly in "excellent health." The average testosterone decline experienced by men – about one percent per year after 40 – is more likely a by-product of age-related conditions like obesity and heart disease than due to the passage of time, the study authors say. While the finding remains controversial, as some experts say declining health is an inevitable part of growing old, "keeping fit, preventing age-related illness, and avoiding excess weight can likely prevent falling testosterone," says Dr. Ronald Swerdloff, chief of endocrinology at Harbor UCLA Medical Center.