How old was O'Brien when he wrote the book? What does the remembering turn into?

From the text:

I'm forty-three years old, and a writer now, and the war has been over for a long while. Much of it is hard to remember. I sit at this typewriter and stare through my words and watch Kiowa sinking into the deep muck of a shit field, or Curt Lemon hanging in pieces from a tree, and as I write about these things, the remembering is turned into a kind of rehappening.