How old?

This is a difficult question to answer. Life expectancy varies greatly from wild to captive reef. The best estimates are in the wild.
The difficulty w/ estimating life expectancy is determing clam age to begin with.Unless a clam is obtained small <1inch it's tough to accurately estimate it's age. Different species grow at different rates.
If you do a search you might find your answers. There are members on the boards w/ clam experience >10+yrs(same clam).