The age of onset of a cognitive disability seems to effect the person with either getting stuck with an obsolete identity or having a better identity.
People who have a history of functioning normally will probably continue to believe that they still can. Lots of evidence to the contrary rarely affects their sense of self. A few changes in their methods of doing activities may be negotiated.
People with a history that suggests that they have never functioned normally and probably hit a ceiling in their cognitive development during their adolescence have a different problem with their sense of self. Their history is apt to include failures to meet normal expectations with persistent criticism and / or punishments.
For someone with a cognitive disability who has never functioned higher, they see the world very differently from a person who used to function higher. They think they have a normal cognition and everyone else who are functioning normally are abnormal and are running around like mad lunatics. They may also be oppositional as they do not perceive the need to follow repeated instructions, warnings or other sound advice. Understanding carers are required to avoid preventable negative consequences.