Why everyone in Guardians of the Galaxy sounds like they're in a PG-13 film

The reason that the language in both Guardians of the Galaxy movies is frequently juvenile and all profanity seems limited to the constraints of a PG-13 film is because we're usually hearing it as Starlord hears it through his universal translator (mentioned onscreen in the first movie's mugshot scene).

Starlord was taken from Earth when he was eight. He probably stopped actually learning English shortly after, since it's unlikely that he had anyone else to actually speak it with. As such, his language (and all language he hears) is going to seem juvenile. Additionally, any profanity he knows probably came from movies and TV shows he saw before he was kidnapped.