shmoel: I believe you have to respect your oldest brother, not all older brothers.
As for people older than you, it says mipnei seivah takum – from an elderly person. That doesn’t mean older than you, it just means someone who is old. I’m not sure what the exact age is.
As for a kohen, I believe it applies for all ages. I remember reading years ago in the book on R’ Mendel Kaplan (who was a kohen), that his Rosh Yeshiva hosted the young R’ Mendel for many Shabbos meals, but would never ask him to pass anything at the table, because he was a kohen.