As for the question about Buddhism, it doesn't really have a lot to do with the age of the civilization as much as the agressive spreading of that civilization. Buddhism may well be the most powerful religion in China and China may well be an older civilization than Europe, but China has never been as aggressive about spreading that culture, through either force or trade. Early in their history, they had no interest in sharing their civilization with other cultures that they considered barbaric. Byt he time that attitude changed, we barbarians had eclipsed them in wealth and power. If that attitude had been different early on, we might well all be Buddhists.
This leads, by the way, to another factor in Christianity's spread. It demands (or has been interpreted to demand, anyway) aggresive proselytizing. Buddhism has never, so far as I know, been interpreted in such a way that demands it's adherents to convert others to it. Christianity has been. Combine this with the spread of western culture over the centuries and it owuld be mazing if Christianity WEREN'T such a major force in the world.