One thing that will be very interesting to see is how Netflix chooses to film the seven movies/series. (Not the same as release.)

Aside from potential minor conflicts with the actress who plays Jadis, there is no reason MN and LWW can't be filmed simultaneously. Then you have LWW -> PC -> VDT -> SC -> LB with the Pevensies, Eustace, and Jill.

But HHB is a bit of a wild card because that's the oldest age given for the Pevensies per Lewis's timeline. That and the end of LWW. 

If they choose to go with two sets of Pevensies (for the end of LWW and HHB), then they can film HHB any time they want. But if they decide to just have one set of Pevensies (movie magic for end of LWW), then HHB will need to be filmed last. And at that point, you'll want them to age as much as possible. They don't have to be as old as is suggested on the timeline, but certainly old enough for Susan to be courting Rabadash. I'll also mention there's no way they'll leave High King Peter out of it.  

I'm personally open to ageing them up a little bit. I think personally having actors that fully represent the ages of the characters in the books will always be problematic when working with live-action.

Although as for suggestions for casting the Pevensies - I only have one. Beau Gadsdon.

https://www.imdb.com/name/nm7917803/

She was born in July 2008 so will be 13 later this year, which would currently match up perfectly with Susan, but I do think Gadsdon still looks so young and I can't imagine a Susan looking that young, especially when her character has so much maturity and has to come across older than Lucy - and I can't imagine Beau Gadsdon looking after a younger sibling.

But regardless of age, she is a great young actress, who's appeared in The Crown, and Rogue One.

These are my thoughts in only the book "The Lion the Witch and the Wardrobe" 

I think that Lucy should be a small child maybe seven or eight because as @rose said the reaction and the belief just won't work unless she is a small child. It would also make sense because if someone younger than you says something unbelievable you are less likely to believe it because small children are known for play make-believe with things that aren't there. I would also assume it would make her encounter with Mr. Tumnus more realistic. After all, what child older than 9 or 10 would follow a stranger into the woods to their house?  

I think that Edmund would be a little older than Lucy maybe two years about 10/11. It would help with the scene with the White Witch because for the same reason as Lucy, it would most likely be that they  would be young and naive so it is easy to trick like that. Edmund is a different story though because the White Witch did not only trick him with his age but with his weaknesses and vulnerability. The reason I chose him for 11 also is because of his rudeness in the beginning of the book to Lucy. He would have to be at least a few years older than her to sound believable to Susan and Peter when he claims Lucy made it up. He would also have to be equally mocking to Lucy because it would hurt more if someone close to her age that maybe she thought was going to understand, but then betray her. 

I think Susan would be at least 13. It would make sense for Susan to be 13 because any younger than that would probably not be the best person to handle with a bow and arrow. (I don't know why they gave Edmund a sword) She would also be 13 because she is a responsible older sister. She would need to be older than Lucy, a little more than a few years to have Lucy gain a sense of security in her. All the same she is most of the time making decisions with Peter, making her seem less like Peter's younger sister.  She is always protecting them with Peter and takes on a Motherly role. Not completely but somewhat because she is always wanting to do the most practical choice. 

I think Peter would be about one to two years older than Susan maybe about 14/15. I think that because like Susan he takes on the fatherly role for all of them. (except for Susan) He acts like he is in charge which makes me think he is 15, a little more older than all of them. It would make sense that in Prince Caspian then that he turns 16, going into adulthood (18). It would also make sense because at the age 13, you are probably not even close to ready to lead a battle, let alone be prepared for one. I think the closest you could get to that age would be 15, being older but not to old to be classified as "grown up". I also think he would be 14 because that would make him more of a child than an adult, which would explain why he is not used to being responsible for these kids, because he is not responsible yet. It would also explain finding Aslan as a father figure rather than a friend figure, because all the responsibility was laid on him as a child when they had to leave home in the beginning of the book. He could be 14 because it would explain his closeness of age to Susan and how he lets her make decisions also. 

@kingedthejust I don't know if you saw earlier in the thread, but Rose-Tree Dryad shared this link: Narnia Character Ages

Going by Lewis's own timeline that he put together after writing the books, plus some internal evidence from the stories themselves, we do have the exact or approximate ages of nearly all the main human characters throughout the Chronicles. In The Lion, the Witch and the Wardrobe, when they first enter Narnia, Peter is 13, Susan is 12, Edmund is 10 and Lucy is 8 — so that's pretty close to your estimates.

I'm hoping the people working on Netflix's Narnia will be aware of this too, and cast the characters so they look and act about the age they're meant to be in each story. As you say, Lucy's role in LWW in particular won't work if she's much older than about 8.

I wonder if Walden's excuse for aging up the Pevensies and Caspian was that Caspian and Susan could have a romantic relationship. 

I am not oppose to age up the children. It would be if they had a good reason.

Posted by: @jasminetarkheena

I wonder if Walden's excuse for aging up the Pevensies and Caspian was that Caspian and Susan could have a romantic relationship. 

I am not oppose to age up the children. It would be if they had a good reason.

I think Walden's excuse would be a perfectly valid one - that they didn't.

Georgie Henley was 8 years old when they started filming LWW (the exact right age) and was even younger when they originally cast her.

It's hard enough casting very young children for an acting role in a major production, and your hands are tied by all sorts of employment laws about working with children... But the biggest problem is that it can take several years between casting and production, and then another few years between production and release, and then another few years to spin up a sequel.... all by which time your children have aged 5 or 6 years.

EDIT: in fact, here is a direct quote from Anna Popplewell that sums it up nicely...

"i was 13 when I had my first audition for Susan; 15 when I made the first movie; 16 at the end of the shoot; 17 when it was released; 18 when I filmed the sequel, Prince Caspian, and now I’m 19." 

- Anna Popplewell, May 2008

https://www.dailymail.co.uk/home/you/article-1022168/Anna-Popplewell-lion-8217-s-den.html

I've been keeping an eye on the published casting calls over the past few months to see if anything comes up that remotely resembles a Narnia production. Sadly I've seen nothing at all that even comes close, nor anything that could even be a potential covert casting using a production alias.

Anyway, amongst my searches, one of the things that I have noticed is that in alot of the casting calls for child roles they say:

"must be aged 9 (by a certain date) for licensing reasons"

All children in the UK under the age of 16 need to apply for a License to take up acting jobs in any production, but I'm not quite sure why Age 9 is the lower limit. I would guess though that below 9 years old the licensing process becomes even more difficult so lots of productions try to avoid it.

Anyway, it was notable that in the recent Harry Potter TV Series casting calls, the lower age limit was also 9 years old.

Might be something to keep in mind when it comes to Narnia when they finally get to the casting stage.

Well, canonically (if Lewis's later timeline is considered canon... there's a debate in itself!) the youngest child character from our world in any of the books is Lucy, who is 8 when she enters the wardrobe in LWW. According to the same timeline, Eustace and Jill are a year younger than Lucy, but as we don't meet them until two years after LWW (in Earth time!), they're 9 that year and Lucy is 10. So having a lower age limit of 9 in casting shouldn't create much of a problem if they cast the child characters as approximately the age Lewis intended them to be. Which means there's technically no need to age up any of the characters, except Lucy very slightly. Whether or not the screenwriters decide they want to age them up is another matter... 

Casting child actors would be idea, especially for the Pevensies. There is technology of de-aging teen actors, though that would look awkward.

So for casting, say for Pevensies, Peter 12-13, Susan 11-12, Edmund 9-10, and Lucy 8-9. Then there is the adult Pevensies at the end of LWW and in HHB. Then in LB, Peter  21-22, Susan 20-21 (though I don't know if they'll include her in that), Edmund 18-19, and Lucy 17-18.

There are some factors to consider:

1. At the beginning of LWW, they are children and adults at the end. They would obviously need adults to portray them in HHB. If they are going in chronological order, they could use the same adult actors from the end of LWW in HHB. If they were going in publication order, it's possible to use the same child actors for HHB for the Pevensies, because they would have aged up by then.

2. Three of the Pevensies also make an appearance in LB. Though I honestly don't know if they'll decide to include Susan. If it is done chronologically, the Pevensies would have aged up by LB, so they could use the same child actors. It could even be the same if it was done in publication order.

So there things to keep in mind.

When casting a young child in UK, any production must take notice of the laws protecting them from child exploitation.

Try this chart:
file:///C:/Users/corac/Downloads/nncee-child-performance-regulations-table.pdf

I worked as stand-in for an 8-year-old in a small film studio in 2018. She had a teacher (3 hours a day), a family chaperone, a private room to rest, study and play in, and maximum number of hours (the chart says 3) rehearsal and filming.  A 9-year-old could have worked longer, but would have looked too old for the character, and would have been 10 by the time they did their second series.

These casting calls indicate that they'll want the child actor to be 9 by the time they start filming. Depending on the role, It may be the younger the better (eg if casting Lucy, or starting a set of series, such as in the Potter books) They must have 3-5 hours a day of education (and cope with being taught this way) , and not rehearse/perform more than 5 hours a day. 

If I were casting the Pevensies, I'd want to know how quickly any older siblings grew, look at their parents' height, and consider how tall the the child is for their age. (background material for Walden LWW shows them all growing a lot, especially the younger two) 

An argument, in my opinion, for aging up the Pevensies are Peter's two hand-to-hand combat scenes. Even if Netflix doesn't put as much emphasis on those two scenes as Walden Media did they are still integral parts of the stories, especially the one with Miraz, and will have to be taken into account when casting. I know absolutely nothing about fencing and sword fighting but I have a weakness for films that have those types of scenes and as a viewer I want them to be believable, they are not going to be believable for me if they cast a boy who is 12 while filming LWW and 13 in PC. (I have four brothers in their twenties and two nephews in their late teens, five of them are well over 6 feet in height and none of them would have been able to physically resist an average sized man or an exceptionally tall woman at the ages of 12 or 13.) I'm not saying Netflix should cast children who are a lot older than the timeline suggests but the slight aging up that the BBC and Walden Media did for their respective adaptations has always worked for me.