I'm only here because we're all here. But summer child seems like the most prosaic insult possible, if one can even call it that, and it serves as a sort of useful shorthand. With a certainty approaching 99%, I think there is something better to be annoyed at.
I don't think it's super insulting or anything, it's just a condescending swipe. When you take a condescending swipe, you set a ceiling on the tone for everything that follows.
