There's a lot of commentary I see that "Krile feels older" or "Krile ought to be older" or "no way Krile's only 22" and so forth. But like I said in the post here about character ages being all over, and characters like Nanamo being younger on purpose for their plotlines to work, Krile's younger age and seeming maturity does make sense.
Replaying the end of Dawntrail base MSQ on the last of my four alts, there's a comment that Krile's very sensitive Echo has been active since birth. We always knew she'd had it since she was very young, but that it's been active in some form since infancy is new.
(The first Encyclopaedia Eorzea said "she discovered" it when age twelve, so is this lore adjustment meant to be "that's when they finally researched out what her gift actually was"?)
Imagine growing up an empath, feeling everyone's emotions - especially when directed at you - and able to randomly without warning see scenes from peoples' pasts. In a world where this is not common, and where the Echo was all but forgotten - indeed, Krile may have been the trigger to begin seeking out such powers among others, leading to the discovery of Minfilia's own gift and what it meant.
We finally have a short story in Chronicles of Light vol 2 of the day Galuf gifted Krile her distinctive hood, her first meeting with Ejika...and descriptions of how very difficult it was for Krile growing up. The Isle of Val was a haven due to how sparser the population was compared to Sharlayan, and most of them adults. She had no real friends her own age.
EDIT: She did spend time in the orphanage Galuf ran, early on, but it seems it shifted to her spending time with him directly and on the isle, especially when things became more difficult due to her Echo.
So this is an orphan girl raised by scholars, with few other children to interact with, while struggling to control an ability no one else understood at the time. She had to learn to control herself, to handle things no child should be asked to.
(Also why Minfilia was her "dearest friend" despite their distance and the 5 year age disparity; those were the kind of relationships young Krile had to keep herself from being overwhelmed, and who else could understand what she had been going through her whole life?)
And Krile still has her doubts, her fears, her insecurities. She expresses many of them honestly over time. She isn't actually a woman who hides her emotions. That she doesn't act out the way some of our other early twenties friends and companions do speaks very much to the differences in their upbringings, but also in what Krile had to deal with since birth. It makes sense she approaches things with far more calm and reserve, and comes across as more mature for it, given that history. She relies on her scholarly background to get her through a lot.
My last point is, we know she went to the Studium at the same time as the Leveilleur twins. She went, presumably, at the usual age for attendance, while they of course began very early. Krile is described as a mentor for them, the upperclassman who looked after them in their cohort.
And, frankly, being asked to look over a pair of pre-to-early adolescent prodigies would age anyone. Look at what they put their childhood babysitter, Urianger, through, for another example!
