In 2018, 56 is the most common age for an American (U.S.) Stanley who is registered male with the Social Security Administration. It is the 285th most common male first name for living U.S. citizens.
― Anonymous User 10/11/2018
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I believe this name was often adopted by immigrants as an English "translation" of the Slavic name Stanis³aw / Stanislas / Stanislaos / Stanislav.