As a child I watched tiny fiddler crabs living in conical shells, and many years later I find that people study fossil turritellids. So now I wonder: how old are shells, typically, that you see on beaches? Do they wear out after a year of sand abrasion? 100 years, a million? Is it known?
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
Ages of shell as a piece can be checked or counted.
1) Examine the shell's ridges with a magnifying glass.
2) Tabulate the number of ridges. You can approximate by number of cell per unit length.
3) Divide the total number of ridges by 365. Each day the little mollusk earns a new ridge, thus total will give you its age.
But, that is age of one shell. If you are asking the age of all of them in a beach, my guess would be as old as the beach.