O.k. I will ante.
So, ante up framebuilders!! Hey, why spend $2,500 bucks for something that is only good for a couple of years? If I want to do that I can buy a car. Richard Rose (Toledo, Ohio)
Yes, this is what it seems. Only a myth used to convince people that they should trade in their old stuff for new.
First thing to say is that anything can break. Stock bicycles are often not designed for the type of riding that they are subjected too. Inconstancies in manufacture or materials can cause failures. Bicycles, even including the modern fragile aluminum and carbon ones are surprisingly reliable considering how much stress they are subjected too.
Basically though, steel and titanium have what we call a fatigue/endurance limit. What I mean by this is that if I design a frame so that normal stresses to not exceed this limit than theoretically it should last forever. The thought that frames become "soft" is completely untrue. The inherent stiffness of metals do not change. Metallurgy is some wacky stuff, big numbers such as high tensile strengths and the like don't tell the whole story. Suffice to say. Ride your steel bikes with confidence, no matter how old. I would check occasionally for hairline cracks in the paint or corrosion "rust" showing through but if all that is fine, ride.