A juvenile yukkuri.
A koyukkuri is born inheriting some degree of knowledge from their parents but they generally understand basic survival like eating and shitting and also "easy stuff" like greetings and rub-rub. They're born with the capability of speaking (albeit slurred) and can usually make basic conversation.
Still, just like a human baby a newborn koyukkuri usually can't survive without a caretaker. Their teeth are soft and is incapable of chewing hard food. They can't hold their shit and need someone to clean them up. And most notably they're extremely fragile even by yukkuri standards and can die from just about anything. This is compounded by their general lack of a sense of danger and will often recklessly attract the attention of animals and humans who might kill them on sight, sometimes even getting their entire family killed.
While koyukkuri mostly refers to baby/juvenile yukkuris, older "teenage" yukkuris are also referred to as yukkuris. The physical and intellectual capabilities of an older yukkuris varies by the education given by their caretaker, though most generally no longer slur. Older koyukkuris raised by smarter wild or stray yukkuris tend to have good survival skills and can often be seen capable enough to raise their younger sisters if by any chance their parents died. Conversely yukkuris raised in a pampered lifestyle can grow up not learning any survival skills and will inevitably die once they need to live on their own (unless they leech of a husband or clan that's too nice for their own good)
Use the koyukkuri tag both for baby yukkuris and for older child yukkuris. The main thing differentiating older koyukkuris and adults is that an older koyukkuri would still live with their parents or siblings.