The Producer of all the Harry Potter films, David Heyman, believes the child actors portraying the three main roles will probably have to be replaced in the fifth or sixth movies since they aren’t getting new movies out every year, and the kids will outgrow the characters.
However, the new director, Alfonso Cuaron, believes they can go the distance.
Unless JK Rowling is delayed in writing #6 and #7 I tend to agree with Cuaron. As long as they can keep to a year-and-a-half pace (Which they have done for 2 and 3), the kids won’t age too quickly. Even if there is a growth spurt, all teenagers go through growth spurts at different ages.
Currently the actors have aged 1 year extra than the characters. At a year and a half pace, they will have aged only 3 years extra. A 20 year old can play a 17 year old. (TV shows do it all the time)
IF there is a growth spurt that is just too unacceptable…use CG animation to shrink them. ALA the hobbits in LotR. Don’t try to tell me the movies won’t make enough money to justify it.