Continuity mistake: After the children are transported to Narnia, Edmund's hair is significantly shorter than it was while they were in England.
Revealing mistake: Eustace, in dragon form, is flapping his wings with all his might as he tows the ship. But in the overhead shot, while he continues flapping, his shadow on the water below is completely motionless.
Continuity mistake: When Edmund is drinking water from a cup, he lifts it up to his mouth but in the next shot (from the side angle) it is lower down.
Deliberate mistake: When Gale and Lucy see the Blue Star for the first time on the Volcanic island, on the second shot of the beach, the sea and the boat and star, the view from the beach, it is a still picture, the waves are not moving. All of the characters are moving but the sea is perfectly still.
Continuity mistake: The seventh sword that Eustace had placed on top of the six swords is missing in the scene of the three lords waking up at the end. (01:33:17)
Continuity mistake: In this film, Prince Caspian speaks with an English accent. However, in the previous film "The Chronicles of Narnia: Prince Caspian", he spoke with a Mediterranean accent.
Other mistake: Eustace's diary seems to be perfectly fine as he was making entries on board the ship despite it being fully exposed to the sea water.
Answer: He's suppose to be the same age as in the novels, so about 16. "Voyage of the Dawn Treader" takes place 3 years after "Prince Caspian", so 2306 (Narnia year). He was born 2290 (NY). However, since Caspian is around Peter's age, they chose an older actor to play Caspian to match William Moseley's age.