I'm confused about your disclaimer -- the Norse did cross the Atlantic and settle in the New World. The technology existed with their voyages. Do you mean to ask about the impetus for Columbus' voyage, which was not technological?

Well, the caravel was the first popular ship that could actually handle getting away from the coast and into the strong winds of the open ocean, and it popped up around 1451.

As for navigation: Colombus ultimately used crude dead reckoning, perhaps in tandem with other forms of more precise celestial navigation that were being refined in Portugal toward the end of the fifteenth century -- namely the ephemerides (~1478). Granted, Colombus was an inexperienced navigator and made several hilarious mistakes (mile conversions, the diameter of earth, expected locations), but it ended up working out.

Of course, the bigger problem was actually acquiring the funding to make the voyage. He tried several times in the later fifteenth century with several different monarchs, but in general it was seen as a stupid idea and a poor investment among experienced sailors -- the expected distance was so huge that it'd be tough to keep the crew healthy and fed the entire time. Despite that, Columbus was granted an audience in 1485 (and again a couple years later) to discuss the possibility, but the King of Portgual didn't go for it

It wasn't until the Reconquista ended in 1492 that Spain finally decided to take the bait, desperately motivated by the promise of reestablishing and controlling trade with Asia after it was hindered by the Fall of Constantinople in 1453.

So... for a direct answer, 1451 if one were to get lucky with the ships, 1478 to ensure better navigation, and 1492 to acquire the actual funding. Christopher Columbus hit the nail on the head.