I think we can all relate to being up late, thinking about what you are going to do for a big event happening soon. Whether it's creating plans for the first day of school like Benji is, planning for a big performance, or thinking about arguments long passed that you could've won with one quick witted comment. It's a pretty relatable thought process, and also a neat ending to a book in the sense that one is preparing for new beginnings at the end. That's why the last line from the older narrator Ben: "Isn't it funny? The way the mind works?" is such a weird turn.
For one, we go from the stream of conscious narration of Benji's plans for a successful start to the new school year to an interruption of his older self shooting his younger self down. The tone of "Isn't it funny?" definitely has a mocking connotation to it, and when you take into consideration this statement comes from the older, more mature Ben and is directed at his younger self, it becomes even more apparent. What's more, in the paragraph before Benji goes on about how 15 can mock 14, justifying him mocking his beginning of the summer self for knowing so little, a dynamic which is repeated.
Second, the line "The way the mind works" is an obvious jab at Benji's thoughts. It's almost as if he's saying "get a load of this guy". The obvious critique further distances Ben from Benji, but is Ben criticizing Benji's childish goals for his new semester? Is Ben just insulting himself for the sake of it? Maybe it's up to interpretation. All I know for a fact is that Ain't No Stopping Us Now is a bop.
