My cellular plan includes lines for my wife , my sister (who I am helping out by providing her a cellular line) and my adult, living on her own daughter (same deal, im still paying for the line because I always have).
While my wife and I, use less than 2-3 gb of data a month, my sister (who is 35) uses like 10-12 and my daughter (27) uses like 15-20 (GB). Neither one of them make much effort to be on wifi all the time like I do, and my daughter is constantly streaming youtube, accessing instagram, etc etc on her phone.
There are also people who basically try to use their cellular devices hotspot as their home internet. Doesnt take too many netflix / amazon video episodes of something to run that data total up. Saying all that, they can throttle back to 3g speeds, which is still not "56k".. not even close.