Revealing mistake: Throughout the dance scene between Darcy and Elizabeth, a small earpiece can be seen in Elizabeth's ear. I would assume this is to help with the dance steps. This can also be seen during the dance with Mr. Collins, however it is not as noticeable. (00:39:00)
Revealing mistake: On many occasions throughout the film, Keira Knightley's real, short hair can be seen under her wig.
Answer: According to the book, at the beginning, Jane 22, Elizabeth 20, Mary between Elizabeth and Kitty so 18 or 19, Kitty 17, Lydia 15.
Myridon