I disagree. I think that this song is about a guy who was, "18 and bulletproof" in college, the kind of guy who would go out partying all night and still manage to "be a morning person". Now, he's out of college and having to deal with a steady job that too demanding for him to keep up the lifestyle he used to have, and his friend (the speaker in this song) is urging him not to let go of his youth so easily.
I disagree. I think that this song is about a guy who was, "18 and bulletproof" in college, the kind of guy who would go out partying all night and still manage to "be a morning person". Now, he's out of college and having to deal with a steady job that too demanding for him to keep up the lifestyle he used to have, and his friend (the speaker in this song) is urging him not to let go of his youth so easily.