boomhauer, I ride aluminum and have frames dating from '86. They'll last a lifetime, but there is one thing in particular to look for and that is a crack in the tubing. Normally this will occur where a tube is dented by accident and, far less often, where two tubes join. On the other hand a steel tube with a crack shouldn't be dismissed either.

When aluminum started to become popular there were basically two types of tubing used, 6XXX and 7XXX. Some early 7XXX frames were considered brittle, but while that seems to have been remedied pretty quickly it didn't help aluminum's reputation. Few, if any of the aluminum touring, mountain bike and hybrid frames are made from the flyweight tubing found on the period road racing frames (many of the early aluminum road racing frames aren't either), if that soothes the soul any.

Brad

PS I feel that most sudden frame failures, other than sudden impact, had a 'failure in progress' that wasn't caught early.