Hi Ed,
I've been watching for an answer to your query and haven't seen one, so here
goes.  Lambis species, like other members of Strombidae, do not produce a
thickened lip (and in the case of Lambis, lip spines) until they are fully
mature.  Then the lip thickens, in Strombus, or the spines begin to develop
in Lambis, followed by thickening of both lip and spines.  This is a
one-time process, and once it occurs, the shell doesn't increase much more
in length, though it may increase in weight due to secretion of additional
layers of shell material.  An immature Strombus or Lambis looks somewhat
like a Conus, with a smooth tapering body and a sharp thin lip.
In Phalium and other Cassidae the situation is somewhat different.  These
produce a thickened lip at the end of each growth phase.  Some of them
dissolve the lip before entering into the next growth phase.  Others do not,
and the former lips can be seen as thickened varices on the shell surface,
or on the spire.  So, a thin-lipped shell is not necessarily "immature", but
is simply undergoing a period of shell growth.  However, the shells do get
thicker and heavier with age, and growth does slow down.
Paul M.