A well built aluminum frame will last longer than most people will care to keep it. It's true about the lack of a fatigue limit for Al, that is one reason aluminum frames typically have such large tubes - to reduce the flex thus, to extend the frames life. I'd be leery of about the life expectancy of a small diameter tubed aluminum frame designed for comfort, ie, flex. Alas, the bicycle industry doesn't seem to make any of these...for good reason.