The reason why the Neteyam is still growing up after 15 is because Lo'ak is guessing what the Neteyam face would look like if he grew up with him, but the more he guesses, the more blurred it gets..
After Neteyam's death, loak secretly recalled the term "baby brother" over and over again... At some point, loak began to touch tuk's head and call her "my baby sister"