Yeah, unless, you isolate a shrimp or somehow "mark" them you won't be able to know which one died when and how long they lived etc. Unless of course, if you keep them as all females or all males
I haven't "tagged" any so I could keep track of individuals but I have the sense that they occassionally live to two years old but I think that is close to a maximum age and 18 months is what I estimate is a good run for most of them..