Is that because they're older? Or is it because when 70 yo's were young they could go through college on part time work, buy a house with an average wage, and then have that house skyrocket in value because housing was turned into an investment vehicle?
Dig into those statistics, and look at how much of the €400k net worth is tied up in their homes, homes which they owned outright at 25-27 years old. That's unimaginable today. People that age are living with their parents.
Look at those statistics and compare the median to the average - the distribution is extremely skewed - you could even call it 'unequal'. 'Record levels of inequality' even.
Age isn't as much of a factor as the 'nerdwallet' blog seems to think; and to the degree to which it is, is a natural consequence of having had more time to work and invest during a boom period, then having a certain generation and class of people 'pull up the ladder' after them.