I too believe that how an individual responds to treatment varies with each person.
My PTSD was diagnosed when I was 25 and it manifested from both childhood and adulthood experiences. After about 3 years of therapy (with medication only in the last year) and becoming a Christian, I can say for certain that my disorder is sucessfully treated. Maybe I still encounter some slipups, but I can now handle situations much better than I used to. My mental state is a lot more stable and capable of coping with certain triggers.
There are some cases that may severely impact a child and may be close to impossible to treat, however. Consider a child victim of narcissism, for instance.
I researched narcissism so much I can almost smell it. I read an article that discussed a narcissistic father who wanted to win a custody battle. He totally brainwashed his child so that the child believed the mother was the enemy. The narcissist had done so much damage to this child's mentality that the child himself began to portray narcissistic tendencies. If not recognized and treated early, the child's narcissistic traits may grow to overpower all other traits in this child's personality and the child will become a narcissist himself.
I do know that even if a narcissistic father is physically abusive toward his child victim, he can twist the child's mind so much that the child will still suffer PTSD, but will still develop narcissistic traits and become a narcissist himself. Of course, if the child is able to be treated early then the abuse may be reversible. However, depending on how long the abuse and how young the child, it is possible that the child's treatment will never be successful. An adult narcissist is extremely hard and exhausting to treat.