The age of a cuttle is hard to determine, size means very little since size is directly correlated to how much they are fed, you can have two cuttles the same age but vastly different in size. Size and age are also directly related to temp, the lower the temp the longer you can maintain them, but 18-20 months seems to be about the max, with 10-14 months the norm. I must say though, I have never seen or heard of a Sepia officinalis of 24" total length! Sepia latimanus maybe ....
BL