The book doesn't actually describe Caspian as 13, does it? Here's the relevant quote. 

His white beard turned to grey, and from grey to yellow, and got shorter and vanished altogether; and his sunken cheeks grew round and fresh, and the wrinkles were smoothed, and his eyes opened, and his eyes and lips both laughed, and suddenly he leaped up and stood before them — a very young man, or a boy. (But Jill couldn't say which, because of people having no particular ages in Aslan's country. Even in this world, of
course, it is the stupidest children who are most childish and the stupidest grown-ups who are most grown-up.)

If you want to picture him as the age he was when Eustace knew him, I think the description allows that and it's what I've always pictured myself. 

Posted by: @aslanthelion

I will add that I’m not a religious person, so I’m guessing there’s some Christian symbolism here I’m not seeing. Maybe that will explain things a bit better.

Keeping in mind that the Chronicles of Narnia aren't a strict allegory (i.e. not everything has a direct one-to-one relationship with the bible) and that there are plenty of other references and parallels in the stories that have nothing to do with the bible....

But yeah, I'd say this is one of the more overt pieces of Christian imagery within the books, specifically with relation to the Crucifixion and the Last Supper:

"Then he took a cup, and when he had given thanks, he gave it to them, saying, “Drink from it, all of you. This is my blood of the covenant, which is poured out for many for the forgiveness of sins"

CS Lewis never intended Narnia to be an allegory. Though there are Christian themes in the series.

Before Aslan brings Caspian to His country, Aslan, Eustace, and Jill take the time to mourn for him. Yes, even Aslan mourned for him. It showed that He even had feelings.

What Lewis did was not allegory but to play around with Catholic elements that might have had had resonance to readers -- the thorn, the blood, the "reverse baptism" and the giving of life again, eternal life. I was raised Catholic so I got it as a child but I can see how others woudn't.

I always assumed too that Caspian's resurrection was a special demonstration for Jill and Eustace and not how souls would ordinarily enter Aslan's Country. In The Last Battle the Pevensies just turned up without a special explanation.

@cobalt-jade as he was not Catholic, but an ordinary Anglican, Lewis was using elements common to all Christians. The single thorn mostly reminds us of the crown of thorns put on Jesus' head during his trial. But it's also in place of any weapon for this specific purpose. 

Maybe there are no weapons in Aslan's country? 

@col-klink the BBC got it wrong by using the boy actor instead of the young man, but perhaps there were reasons (availability?). The boy actor always seemed on the young side, perhaps deliberately done for him to be the same as the children? (were they suggesting that we all become children again in Narnia?)  He certainly needed to be older than Eustace, but the late teenage king was more suitable.

Posted by: @coracle

Maybe there are no weapons in Aslan's country? 

Just thinking of the debate we had recently over why Reepicheep is described as wearing a sword when he welcomes our heroes to Aslan's country in LB, and yet he deliberately threw his sword away at the end of VDT ("I shall need it no more") before sailing to Aslan's country himself... I'm not sure what point (excuse the pun) Lewis was making there, if any!  

But yes, the resurrection of Caspian does come down to Christian symbolism, which I remember picking up as a 7-year-old reader despite not being heavily grounded in Christianity at the time. Actually, now I'm a little older and more widely read, I'm thinking the fact that Caspian's dead body appears in the river might be a nod to John Bunyan's The Pilgrim's Progress, with which Lewis was very familiar — Christian and Hopeful need to cross the River of Death in order to enter the Celestial City at the end. (Aslan also remarks at the end of VDT that the way into his country "lies across a river" for those in our world, which is an even more obvious allusion.)

Also, back on the theme of weapons, Caspian and Eustace do have swords with them as Aslan sends them to deal with the bullies — he just says "Now, Sons of Adam, draw your swords", so he apparently hasn't given them new ones in the way he gives Jill a "fine new riding crop" moments earlier. So I think that makes it clearer that the thorn is symbolic, the obvious reference being to the crown of thorns at Jesus' crucifixion.

There's also a children's story I remember from when I was really little, about a lion who had a thorn stuck in his paw and the only creature brave enough to pull it out for him was the little mouse! I don't know if Lewis might have known that one as well. But as far as the Narnia series goes, this is the first time we've been shown explicitly that people who have died are brought to life again in Aslan's country, and I guess Lewis felt the need to show it happening specifically through the blood of Aslan, i.e. Christ, the same Saviour who according to Christian theology, shed his blood for humankind's redemption and overcame death in our world too. It's true that we don't see anything like this happening at the end of LB when the other characters enter Aslan's country. But I'm guessing Lewis was simply making a theological point in an imaginative way, not trying to illustrate the way in which he supposed all people who love Aslan are brought into his country. He wasn't always consistent from book to book with everything!  

@courtenay thank you, I really like the Pilgrim's Progress suggestion.

Since they were in Aslan's country, and Lewis had not yet written The Last Battle, all the wonderful descriptions and narratives about the Real Narnia were not yet on paper (no matter what ideas he had). 
Aslan's country, in The Silver Chair, is a physical place, like the place where the Seven Friends find themselves when they arrive in The Last Battle. Lewis hasn't worked out a comprehensive theology! He just puts in what helps us understand the story behind the story.

@coracle Yes, exactly. It bugs me a bit when commentators get hung up on the various inconsistencies between the stories and try a bit too hard to find credible "in-universe" explanations for them. As you say, Lewis wasn't setting out a comprehensive theology, and he wasn't intending the Narnia books to be analysed as if that's what they contained. He was just illustrating deeper points in whatever way came to him at the time. So the resurrection of Caspian gives us another big hint as to who Aslan is, but once we've seen that, there's no need for it to be repeated in any of the later stories — and it might come across as awkward and clichéd if Lewis had repeated it for any or all of the other characters who enter Aslan's country later on.

I just had another thought, by the way — the thorn driven into Aslan's paw could also be reminiscent of the nails used in the crucifixion of Jesus, certainly more so than a sword would be. But again, nothing in Narnia is meant to have an exact one-to-one correlation with anything in the Bible. If this scene reminds us (either when we first read it, or later on) of someone else whose hands were pierced, who willingly gave himself to that fate, and whose shedding of blood somehow leads not only to his own resurrection, but to that of others who follow him... then it's doing what the author professedly intended, and that's what mattered to him.

I think it's also helpful that even Aslan has feelings-

And all three stood and wept. Even the Lion wept: great Lion-tears, each tear more precious than the Earth would be if it was a single solid diamond.

Even Aslan was mourning for Caspian's death.. So it's like Aslan, Eustace and Jill were not denying what happened- that Caspian has died.

When Aslan brought Caspian to His country with his blood, it is the hope beyond death. Obviously Eustace and Jill were confused at first, "Didn't he died?" And Aslan says, "Yes, he has died. Most people have; even I have." Then Caspian explains that he would be a ghost if he were to be appear in Narnia, because he's not living there anymore.

I hope it’s not wrong that I feel bad for Aslan here. I mean, I know it’s his choosing, but it was still hard to see him hurt, even if he seemed okay afterwards.

Posted by: @courtenay

There's also a children's story I remember from when I was really little, about a lion who had a thorn stuck in his paw and the only creature brave enough to pull it out for him was the little mouse! I don't know if Lewis might have known that one as well.

That's one of Aesop's Fables. (credited to a slave called Aesop in ancient Greece)

Lewis definitely knew it, as Lucy mentions it in VDT when they encounter Eustace as a dragon.!

@aslanthelion I'm sure C. S. Lewis meant for you to feel bad about it. I mean, the part about Caspian coming back to life is happy but the buildup with his dead body, the thorn, the music, is supposed to be sad. If you'd really like the religious meaning of the scene explained without a bunch of "buzzwords" that only Christians will understand, my best attempt at a summary is "death is terrible and even when people come back from it, there's a price to be paid." 

Posted by: @col-klink

@aslanthelion I'm sure C. S. Lewis meant for you to feel bad about it. I mean, the part about Caspian coming back to life is happy but the buildup with his dead body, the thorn, the music, is supposed to be sad. If you'd really like the religious meaning of the scene explained without a bunch of "buzzwords" that only Christians will understand, my best attempt at a summary is "death is terrible and even when people come back from it, there's a price to be paid." 

Well bad about it in general is one thing (what with the stuff you mentioned) but I was talking about specifically feeling bad for Aslan. I mean some people might not since it’s something he chose and he’s already everyone’s “rock” in the series, but I at least do.

Posted by: @coracle

Posted by: @courtenay

There's also a children's story I remember from when I was really little, about a lion who had a thorn stuck in his paw and the only creature brave enough to pull it out for him was the little mouse! I don't know if Lewis might have known that one as well.

That's one of Aesop's Fables. (credited to a slave called Aesop in ancient Greece)

Lewis definitely knew it, as Lucy mentions it in VDT when they encounter Eustace as a dragon.!

Hmmm, now I think about it, there are several similar stories here. The one Lucy references in VDT is actually Androcles and the Lion, in which Androcles, an escaped slave, encounters a lion in the wilderness with a huge thorn in his paw. Androcles pulls the thorn out and he and the lion become friends. The story goes on further, but I'll just share the Wikipedia page on Androcles here to avoid spoilers for the twist in the tale!  According to Wikipedia, that story was later attributed to Aesop, but the earliest known version of it comes from the second-century Roman writer Aulus Gellius, who quotes it from an earlier Egyptian writer, Apion, who claimed to have personally seen Androcles and his lion. 

The other fable traditionally ascribed to Aesop is The Lion and the Mouse, in which a lion catches a mouse who woke him from his sleep; he threatens to eat her, but the mouse pleads for mercy and the lion lets her go. Later, the lion is caught in a net left by hunters, and the mouse finds him and sets him free by gnawing through the ropes. Lewis was most likely thinking of that story when he wrote the scene in LWW where the mice (Reepicheep's ancestors, as we learn in PC!) eat away the ropes binding the dead Aslan to the Stone Table.

However, the story I was remembering is different yet again. I've just confirmed it's actually The Lion's Paw, a Little Golden Book — famous American series of colourful picture books with gold foil spines, aimed at young children. We had plenty of them at home when I was a child in Australia, although this particular one belonged to my grandma! I've just found a YouTube video of it being read — won't link to that here, as it's probably a breach of copyright — and it's definitely the one Gran had. It's a much simpler story, in which the lion asks who will pull out the thorn in his paw, and the story goes page by page through a host of African creatures responding "Not I...", because they're all too busy doing other things — until, on the last page, the little mouse says "I will," and she does, and the story ends there. That book is still in print (it's written by Jane Werner Watson and illustrated by Gustaf Tenggren), but I think it was originally published in the 1940s or '50s. It doesn't seem likely, though, that Lewis would have seen it, as it's an American children's book and I doubt he had any of those in his collection!  

It could be that Lewis, in writing Caspian's resurrection scene in SC, was partly thinking of a sort of reverse version of Androcles and the Lion; in this case, a man (or a boy, rather) drives a thorn into the Lion's paw rather than pulling it out. I'd forgotten that Lewis refers to that story in VDT, so thanks for the reminder, @coracle. But whether or not he also had the Androcles story in mind (or any other traditional tales), I'm pretty sure Lewis would ultimately have wanted readers to pick up on the echoes of the Christian story here — the crown of thorns, the nails of the crucifixion, and the shedding of the Saviour's blood to redeem humankind from death.