Leaps and Bounds
Once upon a time, Feb. 29th was considered Superman's birthday. Editor Julius Schwartz and his assistant E. Nelson Bridwell — a historian, writer, and editor of some of DC Comics' great reprint series — mentioned in lettercolumns on occasion, with tongue in cheek, that Superman's eternal youth could be attributed to his only having a birthday every four years.
Panel from Action Comics #1 © 1938 DC Comics.
Script: Jerry Siegel. Pencils, Inks, Letters: Joe Shuster. Colors: Unknown.
The Kryptonian physiology that made Superman a prime specimen on Earth could of course also be cited as the reason for his continued vigor — which it was, to varying degrees. Some of the so-called Imaginary Stories in the 1960s showed him aging as a normal human would, but many other tales, from those set in an Elseworlds realm to the DC One Million saga of the late-1990s DC Multiverse, portrayed him as aging more slowly (or not at all) far into the future. And of course neither the Leap Day gag nor his own Supermanity explains the failure of his supporting cast to age in real time, not to mention the editorial decree that the Man of Tomorrow — in his popularly known, merchandised Earth-One incarnation, anyway — was "eternally 29" until his first hard reboot in 1986. (For the Earth-Two version, time had indeed marched on, while the rebooted version seen after Crisis on Infinite Earths was stated to have begun his career at about 25 and been around in the floating present for about a decade, a timeframe that was eventually expanded by certain storylines until he officially reached at least 37 by the time of the follow-up Infinite Crisis... before last year's second hard reboot.)
Superman the character debuted on the cover and in the pages of Action Comics #1, dated June 1938 and on newsstands at least one month prior. I fervently hope to celebrate his 75th anniversary next year in grand fashion with a slew of articles, links, and other fun stuff at my coming comics-oriented website.
When he first appeared, incidentally, Superman could not fly. That power would manifest later, as did greater invulnerability and heat vision and other accretions to the legend. Action #1's one-page origin for the Man of Steel — then a vigilante figure and "champion of the oppressed" — could only hurdle great distances due to his tremendous physical strength, not outright defy gravity and will himself through the air like a maneuverable projectile. He could only "leap," in the familiar parlance of the 1940s Superman radio serial and the animated theatrical shorts that it spawned, "tall buildings in a single bound."