For the whole world you may be right today. That why I put the question mark in my comment. (Lots of people here are from the US and assume everyone is.)

I do agree that measured by income the world has entered a golden age. But I do not know whether mobility in income quantiles is at its peak: You can have a lot of mobility at a very low level of income, too. And in the stone age people were probably more alike in income, so that quantiles were closer together and more easily shuffled just by random changes. (Not that this is a good or bad thing per se.)