Ted:
 
The longetivity of molluscs depends on the species and varies
considerably.  Tridacnids grow to tremendous size (six feet) and
continue to grow for 50 to 100 years.  In aquaculture, they grow up to 6
- 12 inches in 2 years when the meat is sellable.
 
Most species grow by addition of a new slell layer at the lip.  This
produces growth lines parallel to the lip.  With a lot of study, you can
determine age roughly by size.  In conidae, growth is near linear for
some years, and slows drastically when the shell is near maximum size,
growth is very minor.  The final size is a factor of genitics and
environment as in humans with variation in individuals, and groups of
individuals.  Conus pennaceus live about 7 - 10 years.  Other cones have
different life times.
 
In Cypraeidae, and Strombidae genera like Lambis, when the shell reaches
maximum size, the additions to the lip cease being additions to a
conical shell and make a final varix, where the lip growth turns outward
for Lambis and inward for Cypraea.  In Cypraea, the lip closes most of
the base leaving a small aperture.  In Lambis, the lip forms spines.  In
both cases growth in length ceases except for thickening the shell
and/or spines.  In many Cypraea this takes about 6 months.  Cypraea
tigris live at least 10 years, others live longer or shorter periods.
There is no easy way to estimate their age after the shell is adult.
There is usually a mechanism used to maintain a sense of ballance, often
called an "ear ball"  These grow by addition of layers, and you can
determine age for a species by counting the rings as in a tree,
providing you know the time interval of the rings for a species, usually
determined in a lab.
 
  In aquaculture of abalone, a cross section of the shell can be used to
determine age, if you know enough about the species.  In studies,
addition of a chemical produces color changes, which can be used to
determine the age by noting the number of color changes in the cross
section.
 
    Almost each species requires a very time consuming study to reach a
method of determining age.
 
Aloha,
 
Wes