the declining learning curve
those kids just wait for school/college to get over so that they don't have to learn anymore.
Once you are set into ride called adult roller coaster you practically stop learning new stuff and get bombarded with daily non-sense - you might learn something once in a while.
the curve at which the brain is used from beginning to end is dependent on the paths you choose and paths you are put into (or pushed into). Because we don't get to learn new stuff we get bored to death. The so called 'you use 2% of your brain' becomes 'i just use my brain to know what i do everyday in the most efficiently lazy way'