Arch,
Its a good rule of thumb, but varies with the variety and the environment they grow in.
Rosettes can live longer if crowded, in my experience. It's also a theory of mine that the hen and its offsets can communicate chemically through the stolen regarding flowering. I have had hens flower that appear to trigger all the offsets to flower during the same season when the offsets are still connected to the hen. This is undesirable, as it can lead to a drastic population crash, but is fortunately rare.
Offsets can come early or later as well. Some of this is due to environment and probably some due to variety. I have one semp that has yet to flower over several years, but also has yet to produce offsets. Other rosettes that have not grown well over several years also have yet to produce offsets.
