Greenwire: If the Clean Air Act were human, it would now be entering middle age. But while the statute has moved into a strange new phase of life with the Obama administration using it to take on the daunting challenge of climate change, there’s no reason for a midlife crisis, current and former U.S. EPA officials said yesterday during a symposium honoring the statute’s 40th birthday. Since the passage of the Clean Air Act, EPA’s regulations have produced $40 in public health and …