|MadSci Network: Zoology|
The answer to your question depends on what kind of shells you're interested in dating! If you're talking about shell fossils over long time scales, radiocarbon dating is usually used (click here for how radiocarbon dating of shells is done, courtesy of the University of Waikato Radiocarbon Dating Laboratory).
If you're talking about living animals, it really depends on what kind of animal: Arthopods (insects, crustaceans) grow by moulting, casting off their old shell each time, so you can guess their age by determining what life history stage they are, but your guess will be very rough. Some animals add layers to their shell as they grow (e.g. molluscs), so if you know how fast a particular species puts on layers, you can determine its age. Shelled cephalopods (cuttlefish; also molluscs) add chambers to their shell, so again you can determine age if you know the rate at which new chambers are added.
Hope that helps!
Rob Campbell, MAD Scientist
Try the links in the MadSci Library for more information on Zoology.