welcome to Healthcare Magic.
start usually between 15 and 25 years of age. But cases has been reported in early ages too like 5-6 years of age. Many cases of schizophrenia do not have family history and absence of family history do not exclude its diagnosis.
As your child is already on the medicines which may be one of the reason for current negative test. Many time low dose antipsychotic
medicines like sizodon to control behavior problems in children. You can ask Chennai center regarding their management plan and start it according to their recommendation.
Wish you all the best.
Dr Ashish Kumar Mittal