Sometimes I say something silly to my children like, "You are eight, you need to act like it." Replace eight with the age of the child. The silly thing about this is that when I say it they usually are acting their age, which happens to entail them doing something immature. However my real message is, "You are not acting as mature as I want you to act and I would like you to start acting it." I think my children receive the message with that understanding at\s well.
If I am right then I am using the "act your age" words as a red herring to successfully convey a similar, but different message.
Do I go for accurately conveying my message or effectively conveying my message? Being a parent is difficult for someone like myself who likes to overcomplicate what should be easy matters.