OOP Thestral mistake?

[davidbartmess]

No: HPFGUIDX 63268

     In OOP when Hagrid teaches his class about thestrals, we learn 
that "The only people who can see thestrals are people who have seen 
death".  When everyone arrives at Hogwarts at the start of the year, 
we find the carriages that have always pulled the students to the 
castle, are pulled by thestrals, and this year for the first time 
Harry can see them.  Luna Lovegood tells them though, that "I've 
been able to see them ever since my first day here.  They've always 
pulled the carriages."  

     Now supposedly, Harry was able to see them for the first time 
this year because he saw Cedric die last year, at least that's what 
I'm lead to believe, but the problem is that Harry saw death long 
before last year.  Harry was present when LV killed his mom, wasn't 
he?  Do you have to be old enough to remember seeing the person 
die?  Was Harry in the same room when LV killed his mom?  Possible 
mistake by JRK?  What do you think?