Maybe that's why the "traditional" Auri men's clothing (ie. starter gear) has a midriff-baring shirt and three-quarter pants....
The exact quote from the lorebook (p83): "It is not uncommon for Wildwood children to look up to their Hyuran friends in their early teens, only to rapidly outstrip them around the age of twenty." (it's in the section specifically about Wildwoods which I assume is why they've written that instead of 'Elezen', but it probably applies to other clans as well.)
So, possibly starting to grow taller sometime before that point, but not guaranteed.
And I think Emmanellain is just really immature, so he seems younger than he is. (Though he's getting better by the end of Dragonsong.) And he's got the young-looking face which adds to that.
I'd put together a list of the Garland Tools character data for all the High House members I could find (and a few 'maybes'), so you can flick through that and see the different height settings they've used for the characters. 50-75 is common. All the Haillenarte siblings (but not their father) are height 100, and so are the two definite members of House Dzemael. At the other end is Jannequinard de Durendaire, who is height zero and... an uncertain age, but definitely over thirty, so full height.
(Let me know if that link doesn't work, by the way. I haven't tried sharing a list from that site before.)
I had a look around the different tribes last night (and also grabbed the chance to *finally* do the Dataqi Chronicles FATE chain). There's a lot of Thavnairian gear pieces and the Lv68 crafted gearsets, and even the ramie ponchos. Plus the Shisui gear that the Buduga tribe wear, which at least does come from an Au Ra palace, come to think of it...
I don't mind having multiple tribes in the "Nomad" gearsets since that's based on Mongolian cultural dress in the first place. Different colours seem like a 'game-design-friendly' version of each tribe having its own variation on the design.
On the other hand what does bug me is that the Mol tribe wear red, but Cirina has pink instead.