I guess that depends on whether or not you are willing to trust Lewis' timeline that he wrote out after he completed the books. It's not 100% accurate to what he wrote. For example, he gives Lucy's age as 8 and Edmund's age as 10 on the timeline but in the books he says they're only a year apart. Most people justify this by saying that Edmund was maybe a year and a couple months older than Lucy and at the time of their adventure, he was in that two month period. Others say that the timeline can't be trusted. But I personally think the timeline is a good suggestion as to the ages Lewis was thinking of.

According to the timeline, Caspian was 13 in PC which to me is pretty accurate of his behavior in the book. Young enough to still be thought of as a child, but old enough to start acting as king. VDT takes place three years after PC in Narnian time. That would mean Caspian is 16 when he marries RD.

If, however, you don't trust the timeline, then the age is pretty much what you think it should be. Caspian's described as about the same age as Peter in PC and implied to be younger. (Peter's age as far as I can find is never specifically given in the books either.) And then just add three years for him aging from PC to VDT.

Either way, he was very young.

EDIT: Well this is funny. After typing up this post, I went back and rechecked the timeline. Lewis has Caspian and RD getting married three-four years after they returned from the voyage. So that'd make him 19-20.

But the book certainly implies that they get married before they arrive in Narnia. So there you go fastblondie. It's open to your own interpretation! But the range between 16-20 is probably accurate.

thnk u!!!