When people tell other people to grow up, are they...?

It's not A or B, but it could be C. And yes, some people claim to be "grown up" but are not and use this phrase willy-nilly to stop someone they don't like, but then there are those with quite a bit of life experience, and when they say it, they say it from experience. No-one can expect you to be wise before your years, but there is a dividing line where you learn to be adult about certain things. So, it depends on who's saying it and with what authority.