According to the books in "Prince Caspian" the age of the characters should be the following:
Peter - 14
Susan - 13
Edmund - 11
Lucy - 9
Prince Caspian - 13
According to Wikipedia if we presume that the film was filmed in 2007 the ages of the actors were:
Peter - 20
Susan - 17
Edmund - 15
Lucy - 12
Prince Caspian - 26
The age of Caspian did bother me when watching the film because he seemed to be an adult who didn`t know what the hell he was doing in this film playing a child. Luckily in the next film Caspian gets 3 years older in comparison to 1 year for Edmund and Lucy, so if the film gets out 2 years after this one by then Edmund will be 17 (instead of 12), Lucy - 14 (10), Caspian - 29 (16).
And then we have 3 more films which will probably be made every 2 years, then in the end we will have:
Peter - 30 (22)
Susan - 27 (21)
Edmund - 25 (19)
Lucy - 22 (17)
On the other hand it probably won`t matter anyhow as none of these characters will be participating in all of the future films because of the series specifics in changing the lead characters between books.
Anyway I still think that they should had taken the chronological order in terms of story and not the one in terms of books having been written - e.i., started with "The Magician`s Nephew".