My D18 survived it’s first 45 years with nothing but 13s but, just as we all get frailer with age, I decided to go down to 12s several years ago - not that it seemed to be suffering with the 13s. The need for neck re-sets does, however, seem to be commonplace on older guitars although I’ve never been there myself.

As for adjustable truss rods; I don’t think Martin used them on anything prior to the late 1980s on the basis of their claim that “Martin necks don’t bend”. True to form, mine didn’t bend, it twisted! Nothing a plane and a re-fret couldn’t cure though.