A large part of this is that doctors are wary to diagnose mental disorders in children/teenagers, though. Often, symptoms of mental illness during development are just the brain doing the neurochemical equivalent of "one leg growing faster than the other"—it ends up balancing out.

There is still some element of truth to this, though, especially with bipolar disorder and psychosis. It puts me in mind of the fact that allergies can also manifest around 25... and makes me wonder if there's a connection there.