사춘기야 translates to "adolescence" or "puberty". It's an interesting choice because it's not a word commonly found in K-pop lyrics, especially in such a lighthearted and energetic song.
In "청바지 CBZ (Prime Time)", the lyrics say "Everyday 사춘기야" (Everyday is adolescence). This line playfully suggests a carefree, rebellious, and youthful spirit, implying that they live each day with the boundless energy and sometimes chaotic freedom of their teenage years, even as adults. It captures the song's theme of embracing youth and living in the moment.
Three members of SEVENTEEN made a song about wanting the weekend, and their answer to every problem in it is denim. The verses are the small indignities of a working week: "할 일이 터져 마치 / 뜨거운 팝콘 같지" (things to do keep bursting open, like hot popcorn), and a phone that never closes, "왜 퇴근해도 내 phone은 / 정상 영업 중인지" (why is my phone still open for business even after I clock out). Both verses give up in the same place, on "에라 모르겠다" (ah, whatever, I give up), which is the most honest line here. The nagging gets a shout-out too, once from Mom and once from Dad, and both times it is the one thing exempted from the good mood.
Then the chorus decides the fix is to change clothes: "청바지 / 싹 다 갈아입고 나와" (blue jeans, change into them and come on out). The whole song turns on a pun you hear once you know it, because 청바지 (cheongbaji, blue jeans) and 청춘 (cheongchun, youth) start on the same syllable, which is why the chant afterwards stutters on it before landing on "청춘은 바로 지금" (youth is right now). The last minute drops into a mock old drinking song, "한잔하세 / 바로 지금을 담아" (let us have a drink, and hold on to this very moment), archaic polite verb endings and all. It is an enormous amount of ceremony for putting on jeans, and that is the joke.