No, her reign WAS 58 years because she abdicated the throne in 1948: it would have been 72 years had she reigned until death. It's true that Wilhelmina always marked her anniversaries off her legal majority and actual enthronement (her 18th birthday, in 1898 -- not her father's death. That was why she celebrated her golden jubilee in 1948, not 1940. Had she not abdicated her throne, one presumes that she would have also celebrated her diamond jubilee.
But facts are facts: Wilhelmina legally became queen of the Netherlands in 1890, as a child of 10 -- just as Louis XIV legally became king of France in 1643, when he was not yet 5 years old. Historians date his reign from May 14 of that year, when his father (King Louis XIII) died, until his own death on September 1, 1715.
By the same logic, Wilhelmina's own reign really began on November 23, 1890, when her father (King Willem III) died -- whatever she herself may have personally thought. As she died on November 28, 1962, she would have also reigned for 72 years. I just checked the exact dates on Wikipedia, and found the answer to my own question: it seems that she would not have surpassed the Sun King's record for the longest reign in royal history.
Rather, she would be in 2nd place -- the very position Her Majesty the British queen now holds, having recently celebrated her platinum jubilee and surpassed King Bhumidol of Thailand.
Wilhelmina became queen on the death of her father in 1890, when she was just 10 years old. Had she reigned until her own death in 1962, she would have had the throne for 58 years, years shorter than queen Elizabeth II...
How long would her reign have been, had she not abdicated the Dutch throne in favor of her daughter, Queen Juliana? Would it have been longer or shorter than that of the famous "Sun" King Louis XIV of France? I heard that Her Majesty the British queen has recently surpassed the record of the late King Bhumidol of Thailand, for the second longest-reign in world royal history.