Good news for healthy older men.  Australian researchers have found that lifestyle factors including smoking and obesity, in addition to depression, are linked to declining testosterone levels.  But age is not a factor.  Gary Wittert, professor of medicine at the University of Adelaide in Australia, says declining testosterone levels are not an inevitable part of the aging process, as many people think.  He says, until now it was not known why many older men have low levels of the sex hormone.  Doctor Wittert says few populaton based studies have tracked changes in testosterone levels among the same men over time, as his study did.