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I find this data very suspect. Greece is one of the poorer countries in the EU and somehow it takes fewer generations than far more rich countries like France and Germany? The only thing that makes sense is that they mention the mean salary, so Greek wages are so low that you don't need much to get up into the mean salary range, I guess.
It doesnt matter as much how high your wage is relative to other countries, when i suffices to afford a middle class life in your country.
Germany is particularly bad, because we have a class segregated high school system and the assistance systems to help low income households to get their kids to study at an university are reaching less people and are also insufficient because the rates were not adjusted to inflation. Oh and also thanks to many loopholes in the inheritance tax rich people can inherit without taxation whereas middle class people have to pay taxes. Finally eastern Germany was colonized and sold pennys on the hundred dollar bills to the western German Elites, creating a huge wealth disparity where most people are simply prized out of investing into anything lasting.
You just answered your own question