Her person and disposition at 15

Things change. She stops being such a tomboy, she begins to care about balls and dancing, she puts more effort into her appearance. She begins to appreciate compliments and, Austen is at some pains to make us understand, these compliments really matter to her because she is not a great beauty but only a girl capable of looking pretty.

Something else changes and I stop here to underline this detail because it can slip right by us:

At fifteen appearances were mending; she began to curl her hair and long for balls; her complexion improved, her features were softened by by plumpness and colour, her eyes gained more animation, and her figure more consequence.

Emphasis added as they say. Yes, her breasts and hips grew. She became a sexual being, no longer a child. Austen speaks of these things in a way that seems circumspect to us but is actually direct and to the point. Sex matters and never does Austen dodge its importance.