Yeah, its not the years. Its the miles ... so to speak.
From all that I've been able to find, the concern about aluminum frames is a great example of a simple truth being extrapolated beyond its likely relevance.
The simple fact that gets abused is that aluminum has no fatigue limit. That means that it is impossible to design an aluminum component that will not eventually fail from cyclical stress.
The abuse of this fact comes from ignoring at least three things.
1) Reputable frame builders are well aware of the issues with aluminum and engineer accordingly so that material fatigue failures are unlikely for the normal life of the bike.
2) Builders using steel and other materials may or may not build their frames so that fatigue limits are not exceeded. And if they don't, the steel or other material framed bikes will eventually fail also.
3) The reality is that real world failures are more likely to come from problems in manufacturing (good welds, was heat treatment compromised, do designs properly minimize stress risers, etc.) or from damage in use than from the material used per se.
The simple fact regarding aluminum fatigue limits is insufficient on its own for anyone to know if a frame is likely to be long lasting and reliable.
So look for a frame from a quality oriented company first (You've done that). Beyond that, probably the most important thing to do if you are concerned about frame failure is to carefully inspect the frame for cracks and damage. Maybe bring it to a bike shop and pay them to inspect it. They'll probably know more about what to look for and be better at spotting problems. Other than that, a robust, stiff heavy duty mountain bike frame should be perfectly fine. Though if you plan on modifying the frame, most hobbyists find steel much easier to weld or braze.
