Scientific American
Ancient Bones Reveal Girl's Tough Life in Early Americas
The Smithsonian magazine...
DNA From 12,000-Year-Old Skeleton Helps Answer the Question: Who Were the First Americans? | Science | Smithsonian
I actually can't find anything on this skull on the Smithsonian main site. Perhaps you could link it? They might have a typo, you know. Every other source I find says that Naia is 12,000-13,000 years old.
We can tell the difference between children and adult bones of our species, as well as other members of the same genus. Plus, the brain reaches 99% of its maximum size by the age of 12.
There is immense evidence that there was reduced fertility between the offspring of Neanderthals and our species, such as the fact that the lack of any mitochondrial Neanderthal DNA being present in our species, yet we have some Neanderthal genes in some populations. This means that either female Neanderthals and male Homo sapiens were incompatible for breeding, or that female children of the cross were infertile. I honestly don't know enough about Denisovans to comment, though, I'll do some research on that when I have the time. However, neither of these groups are considered the same species as ourselves (considering Neanderthals such is a wingnut position, and the relatedness of Denisovans to us has yet to be determined).