When a politician is shown to have told falsehoods in the past, it throws their trustworthiness in the present into doubt. It is perfectly okay to change one's mind after further consideration or to make mistakes, but problems arise when either one does not acknowledge that one has changed one's position or it is revealed that one has told an inordinate number of falsehoods. A few lies in one's career are understandable; a plethora is suspicious.
Yes, I think that presidential candidates are hurt significantly by falsehoods told years before their candidacy because these individuals are running for the highest and most prestigious office in the land. When you are running for president, your whole life will be scrutinized and anything negative you did i the past resurfaces in order to discredit your image.
Yes, presidential candidates are hurt by lies that they tell before their candidacy for president no matter how long before their candidacy for president the story was told. Therefore, if Hilary Clinton told a falsehood about Bosnia and Trump discovered it today, this will hurt Clinton's campaign. Hilary Clinton, however, will be able to spin it.
Think about all the falsehoods that Trump has spit out in only the past few months. They haven't seemed to hurt him at all. And while the particular Bosnia falsehood might actually hurt Clinton, it isn't because people expect their politicians to be honest, it's because Trump already has so much momentum.