I'm not an engineer of any stripe, but I do have a bit of experience with bicycles. Based upon personal observation, I'd say that I've seen more broken aluminum frames than anything else, followed by carbon, steel, and titanium. And I've never seen a titanium frame rust into uselessness, either.
Realistically, a properly cared-for steel frame should last most of a lifetime. A titanium frame will probably go at least that long, with less care. Carbon bicycle frames haven't been around long enough for us to know for sure. And aluminum frames are more susceptible to joint failure than any others, as far as I can tell, which is not to say that they can't last a very long time, but that the odds are worse than for other materials.
HTH!