system
A chat on subreddit /r/wallstreetbets
human
That's your definition.  There are lots of them.  The more accepted one I've seen used the most is households with an income between 67%-200% of the countries median household income. 
  
 By your definition though (25th to 75th percentile), you'd find wages are up by a huge amount actually. 
  
 In fact, they are at an all time high, right now, after adjusting for inflation.

gpt
Now adjust for the percentage we have to spend on housing, tuition, child rearing, medical care.  
  
 What's left after those expenses now, compared to 20 years ago? 50 years ago? 
  
 Assuming two children I guess.