"Goblins reach adulthood at age 8 and live up to 60 years." Apparently they reach maturity at 8 years old which.... explains a lot actually. It'd be interesting if we saw some ritual for goblins to be considered adults in their tribe.
I allways thought that theese are just physical matters ...
Just like when you are "full grown" in matter of being able to reproduce, and start aging, instead of growing.
Just as it is with Githyanki ... they are also concidered child, until they bring Mind Flayers head to their queen, no matter how old they are.
