Trying to determine the age of an animal can be difficult. There are certain features that can assist in this determination, however, many researchers will rely on straight length to place animals in a life history category (an age class).  At the bottom of this page you can find standard lengths for various age classes for California sea lions from the published literature. However, with experience you will be able to pick out many of the other features that will alert you to whether an animal is a neonate, an adult, or somewhere in between.