Why does Holden ask Sally to run away with him without thinking his entire plan through?

Holden is a sixteen year old boy. His brain is still not fully developed and this includes his prefrontal cortex, or the part of his brain that makes decisions. He's sitting down talking to Sally about why he doesn't like school and loathes phonies, then he get's an idea that if he wanted to he could just run away, take Sally with him and go out west to find a job. He has this idea and, at that moment it is the best idea of his life, there has never been any thing better than running away and taking Sally with him. He doesn't think everything through, he just asks Sally and can tell that she hates every detail of it. Sally would prefer for him to stay in school and become rich like their fathers, which is the opposite of what Holden wants because he knows his father is a phony. Holden doesn't think any of this when he asks Sally, he thinks he's in love with her one moment and out the next, but he is just in love with the idea of her, very much like Jay Gatsby and Daisy. Right after he suggested running away he regretted it, but he tried again. Sally proves herself to be a phony at this point, only interested in money and looks rather than actual people. Holden proves that he does not think things through before blurting out what is on his mind. Holden's prefrontal cortex is not developed and he makes rash, and bad decisions, because he does not think things through.