He said that in the early 2000s when a culture shift was happening. The earlier generations were not exposed to computers, as a result, they belonged to a different culture that worked on the paradigm of the earlier decades. There were pioneers among them, sure, but these people were an exceptional minority than any tangible statistic. Those earlier generations would normally have issues adapting to, leaving aside building the emerging digital culture as it was very different from what the society had before.

The first generation that got exposed to computers and started that culture shift en masse was Gen X, and even among that generation, only a percentage got exposed to it - even if that percentage is quite high thanks to home computers like Commodore 64 etc becoming prolific at that point in time. A digital/information culture started emerging right from that point, with Gen X and newer generations starting to build that digital culture with every iteration. Over time, that culture became 'the Internet', all things digital and all that they involved, and started encompassing everyone and every generation. Naturally the generations, or, rather, culture that was shaped with the new digital world would find it easier to work inside and even build that very culture itself. Hence the difference.

So while the lines are pretty blurred now, what Zuck said had some grain of truth at the time not because of age difference, but because of cultural difference among the preceding traditional and emerging digital cultures. Now the digital culture has become the norm, and even a large part of the much older generations have been initiated into that culture, leaving aside the paradigm shift that happened. Therefore his statement has lost its validity a long time ago.