i don't think we should worry about the actors too much. consider how long it will take them to make all seven movies... if poa comes out in 2004, and we give the other films at least two years (just guessing here, guys, these obviously aren't real numbers
)... that means gof in 2006, ootp in 2008, movie six in 2010 and movie seven in 2012. if the actors are 14-15 now, by the time the other movies come out they'll be in their mid-twenties. most movies made nowadays about people in their teens have actors in their twenties in them, i guess hollywood just likes them to look older.
this is considering, of course, that the same actors stick with the movies. i don't think they will, even if they do make all the movies... would you give up ten years of your life just making harry potter movies?