I don't think it is accurate to say they NEVER molt until 18 months old. It depends on the age of the chickens when fall rolls around. If your chicks were hatched before about March, there is a good chance they will still molt in their first fall, but may or may not stop laying that first time through. Chick hatched after that and through the summer USUALLY don't. That has been my experience anyway.
Totally agree, there are so few things that can athoritively said to be "always or never"