Why such a point of characters' birthdays?

[Justine]

No: HPFGUIDX 111064

James:
The birthdays of Harry, Ron, Hermione, and maybe Neville, Luna, and
Ginny will probably have some effect on the storyline in regards to
apparation licenses and coming of age (as mentioned elsewhere in the
thread), but aside from that, I'd say just for fun.

Justine:
I had a scary thought.  Let's assume that she's wishing them happy 
birthdays in the present.  That is, "Happy 24th Birthday, Harry 
Potter!"  I doubt we'll see the same for James, Lily, or ::cringes:: 
Sirius; a tribute to the dead (whether on the birthday or on the 
deathday) is more likely for those who have passed on.  Perhaps 
she's showing us who the survivors are, those who live through the 
books and are still around now...  So Neville, Harry, Ginny, and 
Percy live. Ron's came around before the site opened--though I 
honestly can't remember when the site opened--and so did the 
birthday of the twins.  It's unsettling, though, that from the 
opening to Neville's birthday, there were none to mention.

Justine,
impatiently awaiting a birthday wish for Remus...