Child-like characters?

[KageTokage]

For that matter, how do Alphinaud and Alisaie look exactly like they did in 2.0 (aside from receiving new outfits)?

The lorebook noted that Elezen have longer lifespans then Hyurs and also age slower as a side-effect with their puberty happening in their 20s as opposed to the teens.

That said, I think Alphinaud and Alisaie are unlikely to change much by the end of the game's lifespan.