I think that for America, and white america in particular life's non plus ultra is senior year of high school. At the very least country music seems to paint this picture.
For example, students from Korea are studious, exam taking machines and there's no "jock" culture. The smartest cookies are the "jocks". "Jocks" are looked down upon.
In contrast, students in USA seem, at least White America, seems uninterested, unmotivated or believe in studying. Everybody's trying to become an NBA or NFL player or a celebrity. People who excel academically are shunned and looked down upon.
Maybe there's an interesting link somewhere.
contextually it seems like it means "peak"?