I lost my mother at nearly twelve, probably one of the reasons why I have a soft spot for Harry is that mutual huge loss at a very important age. My mother didn't die suddenly though, but after a two year battle with cancer, so that is an important difference.
Memory is a funny old thing. I've known people who are fortunate and have very clear memories from toddlerhood, or claim to have. Others, like me, have flashes of incidents or everyday things from their very early years and more solid but still intermittent ones from about eight or nine or so.
I suspect Harry is in the latter category. He has said that he doesn't have that many memories of his mother, but in his speeches he has spoken of her sense of fun and her compassion so there are probably occasions tucked away in the memory in connection with that. In a conversation with a lady at an AIDS charity she spoke of the warm and cosy feeling when she sat on Diana's knee and Harry said he remembered that too. Things like that remain.
We have to remember too that Harry wasn't just twelve. He was due to turn thirteen that September and was no doubt looking forward to a visit from his mother at school or a phone call. Instead, he'd been through the trauma of Diana's death and funeral when his aunt Sarah turned up at his school with his mother's birthday present.
At almost thirteen you do have clearer memories than at nearly eleven or nearly twelve, but as Osipi has pointed out for several years the two boys had spent most of their time at boarding school and their holidays had for some years been divided between their parents. So William was hardly likely to be as directly in the 'thick of it' in his mother's life as he had been when he was younger and at KP a lot of the time. We will have to wait and see what the documentary brings forth. There may be some surprises. I'll be watching with interest.