I am not sure their theory is entirely convincing, that the bias is as common or deeply ingrained as suggested. I question the validity of the reasoning that a cheaper worker willing to work longer hours really generates more value over time. I see why a hiring manager may think so, but lower defects through higher discipline has a lot to do with lowering cost of ownership. Driving younger staff harder is also not sustainable, running the risk of brain drain through higher churn rates.
Any number of other scenarios could explain the numbers equally well in my opinion, from promotion into various forms of management to aging developers voluntarily leaving IT due to burn out or pursuing more varied, less tech related interests.