Plural forms of compound nouns

[Unregistered]

Hello! I'd like to know how to form plural forms of compound nouns, such as passer-by, editor-in-chief, etc. Why do we say passers-by, but grown-ups? Thank you.

[Anglika]

Hello! I'd like to know how to form plural forms of compound nouns, such as passer-by, editor-in-chief, etc. Why do we say passers-by, but grown-ups? Thank you.

In both passer-by and editor-in-chief, the word that can pluralized is a noun, whereas in grown-up the words are an adjective and an adverb.