(With help from cycling spouse Joan Jones.)
Metal fatigue occurs when microscopic cracks propagate into the metal as a result of an oscillating load. Fatigue has nothing to do with time or age. A bicycle in a garage does not fatigue. Fatigue happens only through use.
Steel lasts a lifetime with very little fatigue. It may bend and may be bent back. A dented tube can still be structurally strong. Steel absorbs vibrations better than aluminum, providing a comfortable ride. It is a bit heavier than aluminum so if weight is a factor then riders look at double and even triple-butted framesets. Unfortunately steel has declined in popularity through no fault of its own.
Aluminum fatigues relatively quickly. Riding an aluminum frame for 7 years is considered long enough. Some manufacturers recommend only 5 years. In May 1995 Mountain Bike Action magazine published an article called "Aluminum Time Bomb". As it fatigues aluminum develops hairline fractures and then cracks. Both can be hard to detect as they often develop on the inside of the frame tubes. Any hard impact can worsen fatigue. In fact, it's not advisable to buy a used aluminum frame because there is no way to see the abuse it may have taken. Aluminum is a harsh ride. Very unforgiving. Of course, suspension is the answer to this, which is part of the reason why lots of aluminum is used in the frames of suspension bikes.
For a nice review of the metallurgy of bike frames check out Scot Nicol's webpages.