According to this census data (https://www2.census.gov/library/publications/2010/compendia/...), 5'9" only gets to the 45th percentile for males age 30-39, and 5'11" doesn't quite cross the 70th percentile. So yeah I'd consider that the middle part of the bell curve, at least if we're talking about modern-era American men.

I'm pretty sure people younger than 30 and older than 39 also use chairs. So you can't limit the calculation to a certain age range.