Autoimmune conditions can stem from viral infections, yes. But most of the time type-1 diabetics have a very weak phenotype of the disease, that is to say, the patient has pancreatic antibodies (specifically beta-cell antibodies) and produces them very, very slowly. According to this source, half of all "new cases" (whatever that means) occurs in adults: https://diabetesjournals.org/care/article/44/11/2449/138477/...