Regarding the kids ages....if I remember correctly, the actors who play Harry, Ron and Hermione were all the same age as the character. Draco Malfoy was some 6 months or so older than his character.
The first 2 movies were made with a year separating them...so in the Chamber of Secrets, they were all still the same age as their characters (save Draco)...they took a 6 month break in filming between Chamber and Prisoner....so Draco is now a full year (give or take) older than his character and the others roughly 6 months older.
Goblet of Fire is due out in November of 2005....which includes another 6 month break...so when it is released, all of the kids will be a year or more older than their characters. By the time Phoenix hits theaters in '07...they could start having issues with the age of the actors vs the age of the characters.
The thing working in their favor is that the characters are in Wizarding School for 7 years from like 11 to 18...and and what other age do kids change the most than from 11 to 18? So...they could probably pull it off if all the kids want to finish it.
Somehow, though, I don't think they will...I'd wager we have a new cast for Order of the Phoenix....which ultimately will be fine....the actors aren't near as important as the script and director in these films, IMO.
Jim