Schizophrenia is pretty interesting this way. It's rare for it to show up outside of a certain age range, somewhere around 20 to 35 iirc.

It's strongly correlated to genetics, and most people are totally asymptomatic, no idea they have it until one day they do.

The story of sudden-onset schizophrenia inducing a psychosis making the patient unwilling to consider treatment is depressingly common.