I have had this discussion with a couple of guys who were not trying to sell me anything. Both of them, long time bike mechanics, insist that steel frames do indeed "go soft" with age. Now I'm 140 lb. soaking wet and I don't flex a bike that much and I think these guys are both seeing the effects of their own increased mass on their old bike.
Could someone reply authoritatively on this matter or point me toward a good source of information regarding the aging of steel (or other material for that matter) bicycle frames?