I have been accused of making my characters sound too old for their age. I won’t quibble, they are probably right. But recently, when critiquing a story by another budding writer. I discovered a key (discovered for me at least, everyone else probably knew it already). I discovered that by making the character more self-centered it made him sound younger and vice/versa more sacrificing and thinking of others made him sound older.
It wasn’t the words they used. The slang, or disdain for adults. It was the internal attitude of the character that indicated his or her age.
By the same token. Making the character aware of and concerned about others makes them sound older.
I know when observing my own children, they lost a lot of their self-centeredness when they reached their early twenties. They came to realize the universe did not revolve around them.