It's kind of one of those "stock things" that get's bandied in an argument. The first party to say it usually has it most advantageously as a retort with the same phrase often isn't as "powerful" as it can be construed that the retorting party is just copying what the initiating party said. The only exception to this is if the retort is substantiated with reason and groundwork as to why the initiating party is indeed "the one who really needs to grow up".Rellik San said:-snip-
But I know what you mean, it's cheap, and often not even necessary or true. Like any other thing in conflict, it's just an assertion of dominance. "I'm the adult here, because I've just told you to grow up, implying you're child like compared to me, and thus inferior". I'm not normally one for Freudian psych, but it can have its uses in places, but it's usually some sort of projection on the initiating party's end, that stems out of some sort of insecurity about their own maturity.
There are some times when "grow up" does need to be used in a serious, sober way. Like if someone is literally acting like a petulant child, then it's appropriate. But simply using it against someone who merely disagrees with you on the internet is just ridiculous.