I hate the "Eren always had a childish mindset" defence

I keep seeing people say this in ending discussions thinking it's a good point for them. I'm irritated and I'm so close to just copy pasting this paragraph on every comment.

"Eren has always had a child mindset"

Yes, that's the problem, we had a four year time skip where he supposedly grew from that, only to have this final episode undo all the character development. This would not have been as big of an issue if not for the complete lack of hints or things supporting that he was still "childish". Throughout the final season, his inner-monologues and actions clearly show he's grown from being that immature, hot-headed, emotional, cry baby Eren we know pre-S4. Even at the near end of S3, we are already shown signs of his character developing. Saying that "he has always been a child" is NOT a defence, it's instead a huge flaw in his character writing.

"He's only 19" is also really annoying to hear.

I am also currently 19, I feel that we are much more mature than people give credit for.