Everybody is unique, but on the whole, there's still the average. Average height, average SAT score, there is even ‘average looking.' From clothing to education to body features, there's always been a standard.
According to 99% Invisible's podcast ‘On Average,' Belgian astronomer/mathematician Adolphe Quetelet discovered what we now know today as the ‘average' when he aggregated the mean chest size of five thousand Scottish soldiers. Consequently, he took his philosophy and applied to other areas such as marriage and human lifespan, forever stamping his law of averages on the world, starting most notably with the Civil War.
The reason we have small, medium, and large clothing sizes today is that Abraham Lincoln needed a way to mass produce uniforms for the Union army. The US military would standardize both uniforms and airplanes in 1926, “the distance to the pedals and the stick, and even the shape of the flight helmets.”
However, with increased manpower required for World War II, the Air Force jettisoned the average American pilot for new planes with customizable seating, later adapted to account for female pilots such as five foot four Senior Air Force pilot Kim Campbell. She successfully flew her A10 Warthog to safety despite getting hit and losing all hydraulics during the aerial raid of Baghdad in 2003 Iraq.
So despite the continued standardization of certain clothing sizes and educational tests, today we are at least more flexible and egalitarian. You still have the option–albeit an expensive one–to order custom-made Nikes and a bespoke suit. The world is yours. Kind of.