When were commercial pilots first allowed to use glasses?
In the US, the department of commerce had an Aeronautics Branch. It was first responsible for civil aviation safety. On February 28, 1927 it published a list of the first physicians, who were qualified to give medical examinations for pilot licenses. 

On December 31, 1926, the Aeronautics Branch issued the first air commerce regulations, that included medical standards. These standards discerned three levels of physical standards one for each class of pilot : private, industrial and transport. The standards included regulations for visual acuity. Private and industrial class pilots were allow to wear corrective glasses.