This page is really good at explaining dissociation, ES. Synthesis of Models

DID can only develop in the first 6 or so years of life, before the personality integrates. Once the developmental stage of personality integration has already occurred a person cannot develop DID. A person can develop OSDD when trauma occurs after this period of time however. That is... the ANP states and the EP states still dissociate, but the sense of identity ... of 'me-ness' is mostly retained.
DID occurs before the identity has fused at all, so one sense of 'me-ness' for all self states is never achieved in the first place.
PTSD can develop at any age, and can be concurrent with a diagnosis of either OSDD or DID.