Kay Peterson cited a report of a 225 year old Arctica islandica,
similar to the maximum ages reported for Margaritifera.  "(Maritimes, a
> publication of the University of Rhode Island's Graduate School of
> Oceanography, in August of 1986.  It was written by John W. Ropes,
fisheries biologist, Northeast Fisheries Center. <

I'm not exactly sure how the age was determined for a live specimen,
since the counting methods tend to involve some slicing of the shell.
Perhaps it was compared to specimens that were sliced.  At any rate,
here's a bit more detail on techniques mentioned:

acetate peels: Slice a shell in two along a direction of interest.  (it
may be good to embed it in resin first.)  For aging, slice along the
direction of growth.  Polish the cut surface smooth.  Briefly etch this
surface with a mild acid.  Glue it to a piece of clear polyvinyl
acetate (a little acetone does this by slightly dissolving the PVA).
Allow to dry and then peel off the acetate piece.  You get a very thin
piece of shell suited fro examining under a microscope, and slight
differences in etching due to the nature of the crystal structure and
organic matrix tend to highlight shell structure.

stable isotope analysis: You may recall from chemistry class that
isotopes are atoms with the same number of protons and electrons (and
so are the same element and chemically the same) but different numbers
of neutrons.  In fact, the slight difference in weight due to the
different number of neutrons has a slight effect on the chemistry,
tending to slow things down in most cases.  The particularly relevant
isotopes for shells are carbon 12, carbon 13, oxygen 16, and oxygen
18.  These are all stable (not radioactive).  As mollusks make their
shells, they take up carbonate ions from their surroundings.  Several
factors, including the temperature and the biochemical details of shell
formation, affect the exact ratio of each isotope in the shell, but it
is largely proportional to the ratio in the surrounding water.  This
ratio varies with factors such as the season and any unusual local
conditions (e.g., high quantities of methanogenic bacteria).  If a
mollusk lives a whole year and deposits shell continually during this
time, it will generally show an up and down trend in the relative
proportion of 18O to 16O.  Carbon ratios are complicated by the effects
of many biological processes, so they tell more about nutrient types
(major groups of plants, bacteria, etc.), but those may show seasonal
variation as well.  Sampling tiny amounts of shell along the direction
of growth and analyzing the isotope ratios will give you a record over
time of environmental changes in the isotopes during the times that the
animal deposited shell.

These have important implications for managing fisheries (including
intensive shell collecting, e.g. for craft factories).  Species with
average ages of decades to centuries will recover only extremely slowly
from depletion.  Larger, older individuals tend to be
disproportionately more important in producing young, even if
relatively small individuals are capable of breeding.  Also, some
species may be protandrous, protogynous, or otherwise have significant
linkage between size and gender.  (Brings to mind a popular saying of
spammers...)  In such cases, limiting the harvest of large as well as
small individuals would be advisable.  Another issue is the role of
natural selection: if all the biggest individuals are selectively
harvested by people, then individuals that mature at smaller sizes will
do better and dominate the population.

--
Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA

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