How old would it be?

[paul monfils]

Unfortunately there is little information available on the growth rates of molluscs, except perhaps for a few commercially important species like scallops and pearl oysters.  You may be thinking that such a large shell must be very old.  That is most likely not true.  I would expect that a species like this could reach this size in a few years.  The actual rate of growth depends on availability of food, average water temperature, and other environmental factors.