Clairtytoo, it applies in the same way that it does according to the belief of one having a unified core personality that creates splits before the age of 6. This theory proposes that in normal development a child begins life with an unintegrated personality, and that their emotional states become integrated in the first six years of life through normalising experiences and 'typical' parental care. If this doesn't occur the emotional states fail to integrate (such as in borderline personality disorder) and in extreme cases, DID.
In the case of DID the emotional states continue to develop their own sense of identity and separateness and dissociation and further splitting becomes the primary response to further trauma, stress, and other life circumstances. Further splitting and creation of dissociated states can occur at any point in life and it isn't necessary for a person to have passed through a certain life stage to have alters of that age. Identity states (alters) can be of any age, any race, any gender: they can be animals, inanimate objects, 'supernatural beings' - anything. They are whatever the dissociating child needs them to be. Even typical integrated people may have aspects of themselves that behave and think more maturely in certain situations. The difference in people with DID is that these mature aspects of self take on a separate, older identity. What they are NOT is actual people. For instance, you don't actually have a 40 year old woman living in your body, although you (and probably she) would likely argue differently! I know my some of my alts absolutely and uncategorically denied that they were anything other than 100% real, actual beings. But they are not - they are dissociated identity states of the person that is me.