I have had a steel bar fail on a three speed cruiser which I pedaled pretty hard. It did go soft and bent upward without totally breaking. I think it was too thin (light) and too wide, but it did fail and it was only about three years old.

Leonard Zinn specializes in bikes for guys who are too big for regular bikes. His estimates of how often to change may be geared for them.

An aluminum frame can fatigue. So can a steel one. However, it won't leave you without steering. More likely, it will just feel wobbly, then you'll stop and find a crack. That's a lot different than a handlebar. I broke a butt brazed joint on a cheap frame and rode home. Various forums occasionally have pictures of broken dropouts, seatpost clamps, etc. and it's more often on an aluminum frame.

A handlebar is a cantilever beam, so it flexes more. It also flexes both ways, which greatly amplifies fatigue.

An aluminum handlebar is not cast, of course, but if it is work hardened and beginning to crack, don't you think you just MIGHT be able to pull it apart with one more good heave during a sprint?

Of course, it's not too important to the present discussion because most used tenspeeds just haven't seen that much use.