It could mean she is mature, but it is usually more negative than than. She is probably saying that the girl is rebellious, and perhaps disrespectful or moody.

Growing up far too quickly, in a negative fashion. She is eight years of age, and doing things she should not be doing until she is at least 18... And even at 18 probably should not be doing!

Not really. Or not normally. It's more often a comment on changing senses of childhood in a parental generation, and on children's self presentation and social attitude - and the widespread desire to be accorded respect and treated as if we are people, even when we are small. https://www.theguardian.com/theobserver/2000/dec/03/features.magazine37 https://letslassothemoon.com/2015/01/18/what-no-one-tells-you-about-being-a-mom/ And I don't agree that it's negative, or not normally. It's often a warm, affectionate response to a little person who is growing up and trying to be independent.