Age of shells on beach
Mollusk shells found on typical east coast (US) beaches can range from days old (the animal that made the shell died recently) to thousands of years old.  Some shells in our state, North Carolina, have been dated as 40,000 years old.  A high number of "seashells" found on east coast beaches are from mollusks that lived in the marsh on the back side of the island.   The presence of these shells on ocean beaches provides evidence of island migration - the island has moved landward over the marsh until what was once the marsh is now the ocean shore.  Once buried in the sand, the shell is well preserved until erosion uncovers it.  Occasionally, fossilized shells are washed up on beaches after having been dislodged from offshore limestone deposits - these shells can be millions of years old.

-Richard -
Carolina Ocean Studies