Superman is a Leap Year Baby!
Comics fans have long known that Superman's extraordinary vitality and youthfulness are due in part to the fact that he celebrates a birthday only once every four years. Yes, today, February 29, is Superman's birthday, and people are celebrating, as this messageboard indicates.
Long a letters page staple, the explanation that Superman ages slowly because of his odd birthday could only appeal to an idiot or a long-time comics reader like myself. If my memory serves, over the years this legend has been added to with sci-fi factoids that reference the planet Krypton, the length of its days, etc. Just the sort of thing to delight a young otaku but guaranteed to annoy the casual fan. My understanding is that the Superman editors (perhaps Mort Weisinger or maybe E. Nelson Bridwell or Julie Schwartz came up with the notion, who can say?) cribbed the idea from Capt. Marvel, the prototype for much of Superman's mythology and extended family. Marvel's birthday is first mentioned as far back as 1943 in Whiz Comics #47 and is firmly established in Whiz Comics #52 (Mar. 1944) --a story probably written by Otto Binder who also wrote many of Superman's greatest adventures. Capt. Marvel in turn may have borrowed the idea from Harold Gray's Little Orphan Annie. Readers were no doubt anxious to understand why Annie did not seem to age like Skeezix of Gasoline Alley and the Leap Year idea is an answer befitting the whimsical yet earnest adventure strip.
In any regard, this year is the 70th anniversary of Superman's publication in Action Comics #1 and there are celebrations planned throughout the year. By my count, since 1938 Superman has had 18 birthdays...
Superman is in good company, as this site notes.
Mark Evanier delves into the question here, here and here.