How old are each of the main characters in lore?

[Lauront]

Estinien's reaction to her was funny at least.

[Slatersev]

Maybe he doesn't feel old anymore, what with the new body and all. You're only as old as ya feel, and a lot being old in the conventional sense is the way your body feels.

He could also be playing out of the, "Smart of War." He puts on this air of happy-go lucky teen to put you off guard, but then he be using Hallowed Ground whenever he wants, and shooting you with meteors. /nodnod

That's actually it more or less. If you talk to him after certain quests he mentions he feels younger then he has in a very long time and naturally points to being in a much younger physical body as the reason. Couple that with getting a second chance at life after he spent over a century absolutely certain he was gonna die at the end of his quest to stop the Rejoining and it makes sense that he much less serious at the moment. Regardless of how serious a threat Fandaniel is, he isnt something Graha toiled away for a century physically and mentally to stop.

Not to say its all perfect writing, I do agree with the general opinion here that they overdid it in 5.4 with him. But I thought he was fine in 5.5, having a fanboy moment for somebody not the WOL was charming for me, made it feel less like a specific obsession with us and more that Graha is just a major history nerd, so when he gets a second chance at life to also meet all those historical hero's he read about he's gonna fanboy some. Other then that he was treated like just a regular Scion, and that works, speaking for myself of course.

[Eloah]

As someone already pointed out, there are already canonical ages for a majority of the main cast.

According to the Encyclopaedia Eorzea volume 1 - At the begining of ARR:

Alphinaud and Alisaie are 16
Urianger is 29
Thancred is 32
Minfilia is 27
Y'shtola is "Forever 24" (She's probably actually around Thancred's age, since she has a younger sister who is 26)
Lyse (as Yda) is 25
Papalymo is 42
G'raha is 24
Arenvald is 19
Estinien is 32

These are their minimum ages plus your headcanon timeframe for how long the story actually takes place.

There are only a few things I would add to this.

Y'shtola is anywhere between 33-41, since Papalymo was the oldest of the Scions, and story hints indicate Y'shtola is older than Thancred. Speculation is that she is on the lower end of that scale, since she, Thancred, and the actual Yda, were all close in age, and if she was still alive would be about 30. Additionally, she also views F'lhaminn as an "elder" if you will, and she is only 37. Though this could just be showing respect to someone's parent.

Hien is the same age as Lyse, so he'd be 25 as well.

Ryne is probably 17, but can be anywhere from 16-19. The reason for this is because of story cues, she is close in age to the twins, who were a year older on the first, and because of her model. Older teens, 16+ use the adult models, at least for Hyurs, as those are really the only "older" teens we encounter. For example, Leveva is 16, Ardashir is 17, and both Serendipity and Arenvald is 19. But Honoroit, an Elezen, is 14 and uses the child model. The only older teen Hyur who doesn't seem to use an adult model is A-Ruhn, for obvious reasons.