Is it possible to not get affected by PTSD?

Sure. There are soldiers that come back from combat without PTSD and there are people who go through abuse that do not have PTSD also. Mental health issues are a combination of nature and nuture. So, it's not just what happens to you, but also how you are wired. Sometimes it can take several events to trigger PTSD or maybe only depression or anxiety will result.

 

I was just reading an article about this, and the idea that it has to do with mental "strength" is really a myth.  A lot of it is how the amygdala, the part of your brain located near the base that regulates emotions, processes events.  Since the amygdala isn't fully mature until a person is about 25 (and this is the average- some take longer), events from childhood or young adulthood have a very strong impact on how the brain reacts to stress.  So, like the person above me said, it is a combination of both your childhood and your brain development, your "wiring", so to speak.  The variety in the symptoms is partially cultural, partially related to the individual, which is why there is so much variety in symptoms and their severity.  Trying to overcome PTSD requires a person to rewire their basic survival impulses (because the amygdala is part of these and tied closely the "reptilian brain" which governs survival, like the impulse for people who are drowning to grab onto anything to stay above water), to work through the event that caused their brains to change in the first place, and to overcome an instinctual reaction, and it's about as easy as it sounds, being, obviously, not.
On that note, think about the idea that the amygdala isn't mature until about a person's mid-twenties, and we regularly send people who are younger that that in the military into active combat zones.  No wonder so many veterans have lifelong PTSD.

 

I think the risk of it may be that you might not know you are affected but your subconscious might be and it might just manifest later on. I do think some people mature earlier or more than others though, despite circumstances,so maybe it has more to do with their natural personality and their ability to be more focused on themselves rather than fall victim to their environment or experiences. Probably the best measure of this is the ability of the person to be honest with themselves and how self aware they are of how their mind works so they don't risk just fooling themselves into thinking they are okay when they are not.