Like people, different individuals of a mollusc species may grow at
different rates, and reach different sizes before the growth hormones taper
off.  Also, some molluscs, like bivalves, tun shells, moon snails, crown
conchs, cone shells, limpets, and many others continue to grow as long as
they live, while others, like cowries, egg shells, conchs, and pelican-foot
shells tend to reach a maximum size, and then stop adding to overall shell
size (though they may continue to add to the thickness and weight of their
shells).  As Bill already mentioned, a conch shell is mature when it's lip
becomes thick and solid, whether it is 6 inches long or 12 inches long at
that time.  And it is still immature as long as the lip is thin and fragile,
regardless of it's size.
"Growth rings" cannot be used to tell the age of a shell, except in certain
cases.  Bivalves which live in shallow water, in climates which fluctuate
between frigid and warm, may have true annular growth rings, since they grow
rather quickly during the warmer months, and virtually stop growing during
the winter.  Bivalves which live where the temperature is uniform year round
(either constantly warm or constantly cold) tend to grow at a more uniform
rate.  This includes deep water species in virtually any climate.  Such
species may still experience periods of faster growth and periods of slower
growth, based on fluctuations in food supply or other factors, and their
shells may show corresponding growth lines, but these lines are less likely
to correspond to years of age on a 1-to-1 basis.
All of the above also pertains to gastropods (snail-type molluscs), but in
gastropods there is another twist (no pun intended).  Later whorls of the
shell cover over the earlier whorls, so that earlier growth lines or varices
are no longer visible.  In some forms, like Cassis and Cymatium, evidence of
earlier varices may be seen in the spire.  In many other forms, this is not
the case.  Anyway, again, in such species there is no guarantee that each
period of growth corresponds to a year of age.
Paul M.