Great questions! Don't know the answers except for the following. I frequently help a friend who owns a private car and have helped him change wheels many times. As he often buys used wheelsets, I have never heard the issue of age come up at all. As long as they pass Amtrak inspection they go o.k. Quite some time ago, Amtrak banned the use of rim stamped wheels (wheels with date and serial numbers and other info. stamped into the rims of the wheels) as rim stamped wheels were connected to the cause of a fatal Ringling Bros. derailment. The wheels in the derailment had been overheated by a dragging brake on a car equipped with clasp rather than disk brakes. It was believed that the overheating caused the wheel to fail starting where the rim stamps were. As for flat spots, I do know that flat spots cause extra wear to trucks and truck parts and flat spots on locomotive wheels can damage traction motors both at bearings and windings----I would have to assume that good sized flat spots would shorten the life of a rollerbearing.