Roughly four hundred years ago. In writings from the 16th century there are references (snotty references) to less-than-civil people wolfing down undercooked joints of meat. By the 17th century we start to see references to meat being cooked rare rather than just plain underdone, and eating a bloody haunch began to be seen as a matter of preference rather than one of civility. By the middle of the 17th century, particularly in France, there was a healthy dialogue about whether rare-roasted beef was more favored by the digestion than well-boiled beef, and so on. It's pretty much a straight line from there to the modern sensibility that a steak cooked past medium is a waste of meat.