Thestrals and Harry

When was too young to understand what had happened when his parents died.

Cedric's death hadn't hit him yet. He saw Cedric die, and then right after fight Voldemort, found out a person/teacher he trusted greatly had been trying to kill him, and was sent প্রথমপাতা in less than a week. It was too much for him to process and, since he hadn't processed and come to terms with Cedric's death, he wasn't able to see the Thestrals.

EDIT: For the people who keep insisting that JK hasn't covered this - link was taken directly from her site.

IT maybe a mistake of jkr,

but when i think আরো clearly, i think, maybe it was because his mind was full of confusion that he couldnt focus much on the reality that he had seen cedric die. but during the holidays, he spent nights dreaming of cedric and that fact registered hard in his mind. thats why he could see thestrals from then on. maybe this is the explanation.but we dont know for sure-do we? : )

That occured to me when I was পাঠ করা OoTP. This is a mistake made দ্বারা JKR!!!!

I believe that Harry could not see the thestrals when he first came to Hogwarts (although he should have due to witnessing the killing of his mother) because he was so young when those killings took place, he could not remember fully, which would cause him to not see the thestrals. While this issue was never adressed দ্বারা J.K.R. (as far as I know), I don't think it was a mistake.

In order to see Thestrals, one has to understand what death means. Obviously, an infant could not. Like Bri said, Harry did not have the time to understand Cedric's death but I suppose he had a lot of time during Summer to process it and grasp the meaning of all of it before going back to school - hence his ability to see them when entering 5th year.