People forget that Aerie is a (surface) elf; elves mature at a slower rate than humans. However, Underdark drow - despite also being elves - are forced to mature quickly thanks to their dog-eat-dog culture.
I think you are switching two concepts here ... Its one thing when your body transformation is finaly complete ... you stop growing ... and therefore you are physicaly mature. And its another thing, when society is forcing you to take rights and responsibilities ... and therefore since that moment you are concidered mature.
I dont quite see any conection to models that are used for Baldur's Gate elves tho.
In the words of the senior NCO instructor at cadet battalion: “If you ain’t cheating you ain’t trying. And if you got caught you didn’t try hard enough!”