Many people are moving away from the term mental illness, but a commonly accepted criteria is that any condition listed in the DSM is a mental illness. And the more we look, the more we can find both biologic/genetic contributions AND social/psychological contributions to the development of our mental health conditions (the term I prefer)- which would make pretty much all of them neurodevelopemental. Although both schizophrenia and ADHD have strong genetic underpinnings, identical twins are not always concordant for these conditions. And conditions like PTSD, which have a strong situational/environmental component, don't happen in all people exposed to the same trauma.