Lydia is 17 years old, she would be considered a gentry, which is an upper class.  I would guess that her family owns land and she has quite a bit of money coming in from that.  It is explained that she is wealthy and should not marry below her class.

Here are my costume ideas for Lydia.  I would combine these pictures into one costume.  I think that Lydia would wear grey and rose pink.  I love the two colors of the bottom dresses, including their style.  Even though she desires to have a fantasy life of marrying someone below her class, she enjoys the upper class life she lives.