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That kind of sounds like the opening narration to highlander, but i'm not sure since they're not really aliens (at least not until the crappy sequel) but they are immortals.
Also, the narrator had a weird, metallic, kind of cartoonish voice, like what you'd expect an alien in a low budget tv show to have.
Could be 'Heavy Metal 2000", but was animated.
Could it be Treasure Planet? Or maybe Titan A.E.?
A movie about aliens looking for a fountain of youth that was hidden away in space, it started with a narration about the story, which ended with a line like "It was never found. Until...now.". I never saw more than just the opening, and even then I didn't actually see it, I only heard it, as I was in another room.