If that is your standard on which lives to end then be consistent with it and apply it to every one who's brain isn't fully developed. Your upset because your reasoning looks stupid when applied consistently.

Human value isn't based on their size or development. Oppressing other human beings is based on looks is bigotry.

the human brain isn't fully formed until our 30s

Stupid would be not responding to the point.

But you can't prove they exist.