system
Given the following post, respond as if you're commenting on Reddit:
human
[Fantasy] Why do immortal species have such long childhoods?

I'm not sure I'm asking this quite right, but, ok. Take Baby Yoda.   


His species apparently lives to 900, and as such at 50 he's still a baby. Which seems to make sense.  
Except he's still 50. However much his species lives to, he's still had 50 years of learning and experience and so forth. So why isn't he an adult, by human standards at least? Is he fully mature but trapped in a biologically infant body? Or do Yodas (for lack of a better word) have the inverse of a human infants rapid period where for the first hundred years they're thick as pig shit?  


Likewise elves in D&D, who remain children for a century. I'm sure I've heard the trope a lot before.  


Why don't those decades or centuries of learning and developing show up? 50 years of maturing is 50 years of maturing. Why are you still babbling nonsense and playing with block toys?
gpt
Brains also have to physically mature. Even for humans, it takes years. If brain development is really slow by human standards, then he's not a 50 year old in an infant's body, he's a 50 year old infant.