The phrase "big girl" is often used to describe a young girl who is older or more mature than other girls her age. It can also refer to a woman who is confident, independent, and capable of handling things on her own. Full definition
If you are new here, you might not know that I have been talking about decorating my daughter's big girl room for ages now. (decoratingdelirium.com)
We got her a new big girl bed and she was very excited about it... so that told me that it would not be too hard of a transition. (thestir.cafemom.com)
I think I have tried everything - leaving her in big girl underwear, bribery, discipline, leaving her alone - you name it! (babycenter.com)