It's impossible to disprove creationism, because science is in the business of evidence and proof, not disproof. Science has stacked up thousands of little pieces of information that support evolution and an old earth, from many different sources, and practically none that support a 6 day creation 6000 years ago. Thus, evolution is incredibly more likely, so much more likely that it has, to all intents and purposes, been proven (though in pure scientific terms, nothing is ever proven). If, therefore, we accept that evolution and old earth theories have been proven, then by default young earth Creationism has been displaced and disproven.