Why didn't the Age of Exploration start before 1400 AD?

There are many reasons for this. England and France were at war with each other during the Hundred Years' War. This war took up a great deal of resources. The kingdoms of Castille and Aragon in Spain would not be united until the latter part of the 1400s with the marriage of Ferdinand and Isabella, and their first item of business would be to chase out all Muslim influence from Spain. Three of the major exploring powers—Spain, France, and England—were going through their own set of domestic problems. There was also the relative ease and certainty one had of sending trading vessels to Constantinople, which, as of 1453, was still owned by Christians. No one wanted to undertake a voyage around Africa or across the Atlantic in the hopes of getting to China when one could trade with Venetian middlemen who were already doing business in Constantinople and its vast trade networks. There was also the issue of the Black Death, which had wiped out a great deal of Europe in the fourteenth century. It would take the continental economy generations to recover from the loss of all of these people. In many cases the peasants who survived the plague realized that they could ask for higher wages and this put further pressure on societies.