Half of all lifetime cases of mental illness surface by age 14
There is a very large error that parents often make when faced with the mental illness of their child. They label it as a phase that will go away on its own, and therefore do nothing to treat it. However, half of all lifetime cases surface by age 14, so do not get stuck in the trap of thinking "clearly this is not [insert mental illness], they're only a child" because the fact is, they are just as likely to have the mental illness you might be considering as someone far older than themselves.
Often, puberty will be labeled the cause of certain mental illnesses, when this is not the case. Unfortunately, the illness will often strike at the same time as puberty, making it confusing to determine which one came first. However, if your child is displaying symptoms of depression, anxiety, schizophrenia, borderline personality disorder, or any other mental illness, speak with a professional - even if you think it might just be puberty.