the 737 are new and old.
only the 737-600 are all old.
some -800 are from 2014 and some are from 1998.
Check the upper right corner of the window and you will find the aprox date (attleast year) the plane was made.
Often i have boarding the plane and its not attall been cleaned.
I state that because...
I have on several sas flights out of OSL/STH/COP boarded the plane 3-4 min after passengers left that same plane.