Everything that the scientists know about DID show that the majority of people who have dissociation so severe that they have DID, began in childhood among people who were severely abused. There are a *few* cases among people who experienced other trauma like watching someone get killed.

Adults do not develop DID. It is created in childhood as a coping mechanism.

People with DID have been dissociating their entire lives and have lost time, etc their entire lives.
The alters have always come out, because they have functions; for example: one alter may have been created to do well in school, one alter may have been created to deal with all the anger, etc. The host has no or little conscious awareness of what is happening until alot of work is done to develop communication among the alters. Most true cases of DID do not control their alters nor do they have awareness of everything that goes on because the job of the alter was to protect the host from awareness of abuse or things painful.

People don't just "become DID" in adulthood --- they have always been DID since the initial split.