"Adults also need to know when to speak."
Translation:גם מבוגרים צריכים לדעת מתי לדבר.
The main differnce is that מתי is a question word (requires the information of time) and -כש is a conjunction (gives the information of time).
I think, similar to english you need to use the question word here as in 'adults need to know who to talk to', 'adults need to know where to speak'... Maybe that helps.
@Jenna, כש only gets used to mean "when" in referring to a specific action, as in "when I arrive, I will..." In this case, it has to be מתי because its referring to the infinitive/general action לדבר. There is no subject or time frame specified, and it may or may not ever happen.