"Adult Stuck in Child's Body" Syndrome

In interviewing my brother about our little sister, Marielle, he made a comment that I found interesting and true:

" It’s funny cause we’d actually have conversations like adults, you know, I’m a 23-year-old talking to a 4-year-old sitting down eating, it’s pretty funny to interact. "

Everyone always remarks about how my sister is like a 30-year-old trapped in a little child's body.

I've noticed that many children who have been raised in a predominantly adult environment share this trait. They talk like us because they are constantly listening to us. Is this just a new generation of entirely too mature children or is this something more circumstantial?

Looking for any insight people can share with me on this!