Let's look at real-world examples.
For the horns, let's use deer as an example (just because they're quite ubiquitous). They aren't born with antlers; males grow them later in life. I'd imagine it's somewhat similar for the Au Ra, though they probably have nubs or whatever when they're born since they're basically the Au Ra ears.
While the scales lead many to a belief that the Au Ra lay eggs or whatever like reptiles, the fact of the matter is the Xaela Himaa tribe's description outright says "[roughly] one in every three pregnancies results in twins..." This confirms live birth, as does the fact that Au Ra women have breasts. The rate of twins in the Himaa tribe is also mentioned to be abnormally high, and is by real-world standards, suggesting Auri women give birth to only one child under normal conditions. There is also nothing linking the Au Ra to reptiles of any kind; though the scales often draw comparisons to dragons, and some people will treat them suspiciously as a result of this, there is no link between the Au Ra and any scalekin. (That we know of.)
Au Ra children probably develop rather like human children; rather similar until puberty, during which the sexual dimorphism they are now (in)famous for becomes apparent, up to and including their scaling differences (as males have a patch of scales on their back, while females do not).
Without empirical evidence and a way to obtain said evidence, this is the best I can do.
Any other questions?