Can people develoup dementia in their twenties?

Very rare, but yes: Early-onset dementia can strike in the thirties and perhaps late twenties. This is especially true for adults with down's syndrome. It is quite rare in any case. Chemically induced brain damage from illicit drug/substance abuse can also present as dementia in young people.

Yes: Dementia is CNS disorder that usually is seen in the elderly, but dementia can be caused by brain trauma, hiv, parkinson's and other factors that can occur in the 20's.

Yes: Yes, due drugs, trauma or tumor.