How old are each of the main characters in lore?

[MrThinker]

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.

[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.