Can someone please remind me how demographers defend treating post-WWII generations as being 20 years in length when the evidence is very clear that real genera

Can someone please remind me how demographers defend treating post-WWII generations as being 20 years in length when the evidence is very clear that real generations are much longer? http://www.isogg.org/wiki/Generation_length http://www.isogg.org/wiki/Generation_length I wouldn't care so much if it weren't for having to endure another round of inane punditry.