I know the real-world recommended approach is to refer to the person as if they've always been their current/preferred gender, but there's still potential for Viera to have a very different mindset about gender and how they define it.
With little canon discussion to go on (hence my hoping we'd get a bit more commentary about it) my headcanon take on it for the sake of figuring out how my Viera character would regard it is that while kits are
understood to be potentially either gender they're treated as female until they aren't.
This interpretation was partly based on... I'll have to say my
memory of a sidequest that may have been altered since, because I've just checked it and can't find what I recall. The quest is
Delivery Time in Fanow, where they are preparing for the birth of a child, and I thought there was a point where the baby was referred to as "she" (which would indicate they default to treating all children as female) but now it's all neutral language.
But anyway, if they
did treat all children as girls by default then there might be nothing odd to them about a "girl" growing into a man, and talking about it as such.
I just think it's fun to toy around with when they give the races non-human aspects, rather than falling back to handling it how humans handle the closest equivalent thing.