the Age of Alters ...

Firstly, are all alters formed in child hood? What I mean really is to ask if it's possible for new
alters to form in our adult lives during traumatic circumstances or are they all a product of some
childhood traumas?

In the case of my boyfriend some alters were created out of traumatic experiences, and others were later created post-childhood (mid-teens to late-teens) in order to balance the child alters. It had nothing to do with more trauma being added to his life. It was that his mind was acting childish because it was mainly controlled by younger alters, so when the older and late teen alters were split everything balanced.

Secondly, what determines the age of an alter? Would it be that the child wishes he/she was a
certain age to combat the trauma? Or are alters ages determined by the age the trauma happened?

I wonder if witnessing a long, traumatic death could bring about the creation of an alter? Or would
this 'blocking' be more closely linked with PTSD? Or if DID was already present, could this bring an aging/older alter to the fore as a coping mechanism?

Well, Chibi is nine and cannot age past nine because Johann's father had a heart-attack and died in front of him alone in a motel room when he was eight years old (months from nine, but Johann always claims he was nine). Their mind was so badly traumatized that the memory still bothers everyone in the system, even though who weren't there to experience it that night. So yes, it isn't just abuse that creates alters; alters can be born from traumatic experiences, and those traumatic experiences can freeze an alter at the age they were created. And because Chibi was traumatized that night, another alter who was still able to age, HIM, took over for a while after that night. Thus why Chibi is still nine but HIM is the same age as the physical body.