I think it all depends on the child to be honest. My son is 6, and my PTSD/CPTSD has not been an issue until recently with a really huge break down. He recognized it, and began grilling me on it. I didn't give him the details, but I gave him an over-view and an understanding of what was going on. He seems to be understanding, most of my episodes aren't directed at anyone but myself though.
I kind of go by the rule of thumb that if a child is old enough to ask the question, they're usually old enough to get the answer (In a stripped down version most of the time.) that can be built upon later.
The last thing you want to do is alienate your children, if they sense something is wrong (And children are incredibly good at this) then they will pursue it. If you avoid it, they feel like they're being lied to. If they feel as though they are being lied to, they will not trust you, and it's a slippery slope from there on down to full on alienation.
Just remember, we're both their guides and protectors, if we are trying to heal and be better people; we can teach them by doing these things so that if they ever suffer they can approach it in a healthy way rather than bottling it up, suppressing/repressing, acting out, etc.
I want to be a better person, in part for myself (Very small part, I usually dislike myself.) but for the large part I want to show my son and daughter how to be a healthy adult, so that they don't fall into the pits I did by dealing with things in the unhealthy way I was shown by my parent.
Just food for thought.
Blarney.