My inclination is to choose one age to focus the casting on. Realistically, this will not be the 'child' stage, so one of the adult years - basically, if we choose an episode/age for the casting focus, it's then presumed that we will either be recasting at different ages, or using some really creative makeup to get through.
Adanel is a different age every time we see her, and the audience is meant to notice how much she's aged as a clue that time has passed. We don't want there to be any confusion that she has aged significantly between episodes. So, when she first appears, she's a 4 year old girl. In the next episode, she's 19 with some responsibilities. And then in the episode after that, she's 30 years old (now a wife and a mother) and assuming a leadership position among her people. And then when we return to the House of Bëor in Nargothrond after leaving their story for several episodes, she is now in her 70's, and preparing to pass the baton to her successor Andreth. So...I would say that we pick one, rather than cast her 4 times. The implication being that if you pick what she looks like at one age, that is at least a bit of a guide to also cast (or do makeup/CGI) for the other ages.
Haldan also ages over the course of the season, but the audience will not be relying on his age markers to determine how much time has passed. Honestly at this point, beyond existing, I'm not sure that Haldan will have any sort of story of his own. He'll be a child throughout the main Haladin-centric stories. Haleth will carry the story of the Haladin through the entire season. Haleth will be easier to choose, too - we will want an actress in the 30-35 year old range to play Haleth in Episodes 4, 5, and 6. Time passes there, but less than 10 years total. And yes, Haleth will also appear as 16 and an aged lady on our show, but...those three episodes are the main ones that focus on her and thus would be our casting focus as well.
I agree that Bëor will be the same actor across the first three episodes. He is meant to age from 67 (and a healthy/hale/spry 67 at that) to 93 (at which point he is on his death bed). I think that can be handled with hair and makeup choices, and the actor being careful to move as an unsteady old guy for the later scenes. There are certainly plenty of shows where someone is 'aged up' in that way. How believable that is varies from
Star Trek: The Original Series (ie, not realistic or believable at all!) to Marvel's take on old and young versions of their characters (I found that impressive, anyway).
A Beautiful Mind ages up its main character without recasting as well.
I also am a bit torn over Andreth. We need her at two distinct stages of life: in her late 20's for the move to Ladros and her romance with Aegnor, and then as an older woman (60-70) for the Athrabeth and as a refugee during the Dagor Bragollach. So, my inclination is to cast a 25-30 year old actress, but maybe it would be good to do two castings here? I know you *can* use makeup to change someone's appearance that drastically, I'm just not sure how easy it is to act through all that makeup.
And...because you knew I was gonna go there....
Supernatural has handled this issue in both ways. Typically, if they have a younger version of a main character making an appearance (flash backs, time travel, magic), they cast a new actor. So, we get new actors to play 'young John and Mary Winchester' from before the boys were born, and child actors to play Sam and Dean for any scenes requiring a child version of them. There haven't been too many cases where an
older version of a character was needed, so you can see both choices.
In Season 5, Dean is aged up 30 years due to magic (actor was recast):
Sam ages roughly the same amount of time during a montage, and is *not* recast:
(scene from the series finale, so definitely counts as spoilers if anyone cares about that)
Obviously, the recast was a more believable example of aging, and certainly more sustainable for an entire episode. But also it was necessary to have enough time to 'introduce' the new actor as the established character, so we can see from the dialogue/mannerisms/choices etc that this is the same character. If you don't give that breathing room, the audience might not be fully convinced by the recast. For a series finale, the couple minutes of a montage was not the time to recast a main character, so...