Thread:BlitzGundam/@comment-2010893-20140430163638/@comment-1038387-20140430165028

That's a tricky business, as it involves the Multiverse. Who appeared earlier? Historically, Earth-One Supergirl. Then came Earth-Two Power Girl. Then came the Crisis on Infinite Earths. Supergirl died, and Power Girl continued to exist on "New Earth", with her backstory somewhat retconned. Then, some 20 years after Crisis, a new Kara Zor-El Supergirl arrived on the scene. So in that continuity, Power Girl was first.

They probably had the same birthdate relative to Krypton's destruction, but Power Girl's spaceship allowed her to age naturally, while Supergirl's kept her in suspended animation - she entered at human-biologically "16", left as "16", whilst Power Girl came out at 21-ish.