What is the age of a 22 WMR Marlin 782 micro-groove barrel serial 22755171?

Answer 1

Subtract the first two digits from 100 to get the year of manufacture.  I.E. 19xxx means 100-19=81, so that rifle was manufactured in 1981. So you must have really bought that rifle in 1981 or later,  not 1980! ;)

A serial number starting with 22 means 100-22=78 so it was made in 1978.