OK - really rough overview - don't shoot me for generalisations
Personality disorders (APD, NPD, BPD etc) are considered quite separate from clinical illnesses (eg depression/bipolar/anorexia).
Clinical Illnesses (Axis 1) can happen at any time, in response to life events, or just organically, and can be treated with medication and therapy. And yes, having a marked difference between your inner self and public face is a good way to drive yourself nuts.
Personality disorders (Axis 2) are established by teens/early adulthood, and are long-lasting and incurable (but some are manageable with therapy, but not medication). That's why they're considered disorders of personality, not illnesses as such.
Personality disorders are best described as "people to avoid".
Then there's also acquired brain injuries etc in Axis 3.
So someone with a personality disorder may develop a clinical illness at any time (eg major depression) and many do.
And someone without a personality disorder may develop a clinical illness
But a non-disordered person doesn't develop a personality disorder later in life
(apart from Acquired situational narcissism which = turning into an twat after suddenly becoming famous)
