I’m applying to Stanford EA, and for my optional recommendation, I was considering a peer recommendations as opposed to my boss or arts teacher. Would this be considered immature?
Yes it would be considered immature.
Use your boss or art teacher for sure. Stay away from peer recommendations (unless a school specifically asks for one).
Not a good idea to use a peer recommendation (unless a school requires it, which Stanford doesn’t). Use the optional recommendation if and only if it’s from an adult who can really add something new and impressive. If you already have two stellar academic recommendations, using the optional one for a third wouldn’t be a great idea, but if the boss or arts teacher can speak to a whole other aspect of you as a prospective student (and do so with enthusiasm), that would be fine.
Other recommendations to avoid: those from famous people who don’t actually know you or your work well, from friends of your parents who happen to be alumni of the school you’re applying to, etc.