What's up with Alphinaud and Alisaie (small spoiler)

[Edax]

They are 16 years old, thus not fully grown.
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17 years old in the First

But their bodies are still still in the Source and it's only been like...a week?

[Dualblade]

They'll start growing soon, don't worry. Right when the MSQ ends and we get the epilouge!

Rielle from the DRK quests too, she's right around the age to start growing according to herself in the 80 DRK story. So right when the game ends

[Jandor]

Elezen live about 20% longer than most of the other races, and as such age about 20% slower as well. In theory even at 20 years old they'd still look about 16 or so.