PTSD occurs when people have experienced a drastic and shocking event and have not (yet) processed it properly. Such an event could be, for example, a life threat, assault or serious physical injury after an accident. This event keeps coming back to your thoughts or dreams. So people do everything they can to avoid these thoughts.
You may develop an acute stress disorder after a major event. This can be over within weeks, but it can also develop into PTSD. However, not everyone with PTSD has initially dealt with an acute stress disorder.
PTSD can therefore affect anyone at any age. It often hinders daily life, making normal functioning in society difficult.