What is the life expectancy of aluminum alloy frames?
Material fatigue does happens, it is not caused by time, but due to excessive stress on usage, and/or manufacturing process and/or design fault. If all done under optimal condition, no over stress(e.g. a 120kg person ride it without suspension and under inflate tyre), a aluminium bicycle may last forever.

A bang, a fall, can generate impact that stress the frame weak point. If you are not using it for long time, then the stress is little to none, discounting the stress from introduced from bolts/screw/welding joints/mounting temperature contraction/expansion. 

If you search around stackexchange on the same topics, there is advice that you don't put your bike under the snow : i.e. a concern about welding material contraction different than the frame. But this shouldn't be an issue as long as you didn't move the bike from very cold place to very warm place (i.e., temperature different ~ 20 degree Celsius). 

Consumer association with loaded funds, such as the German Stifftung Warentest, does indeed test consumer bicycle from time to time, especially for Ebike, due to ebikes are more susceptible to weight stress (the additional motor and battery weight).  As some report I read, in 2013, series of ebikes are not satisfied due to the stress test. However, in 2016, many passed the stress test, except for one bike that had a cracked aluminium seat post.

So it is all about how the manufacture learn to build the bike better. 

In addition, it is a myth that titanium will fair better when put on stress : it is all about manufacturing process and design.



About the "metal deteriorate issues" 
- A proper maintain steel frame will not rust. Lube and oil them, you will never see the oxidisation
- Aluminium "rust" will stop when oxidation form on the outer layer. 

In addition,  steel frame WILL NOT rust, if you maintain it properly.



Update from @Barny : 
** Having been the owner of a marin larkspur hybrid 6061 alu frame that has been replaced no less than three times under warranty, each time due to fatigue failure ( i am 5'8'' @ 77kg ) and each frame lasted between 12k - 18k miles, i am of the opinion the lifespan of a 6061 alu frame is around 15k road miles. First failure driveside chainstay split apart halfway along, second failure driveside seatstay split halfway along, third failure crack around downtube / bottom bracket. **