She danced with Mr. Bingley in Ch. 3, so the sisters who were not out yet must have been younger than she. But she also seems to be a friend of Kitty, though I found the reference in a chapter we have not covered yet. (I don't think this constitutes a spoiler, since it's such a minor point, mentioned in passing). If she is Kitty's friend, I would put her age in the late teens. However, we know that Charlotte was a good deal older than her friend Lizzy.

I have always thought the Lucas boy mentioned very young--like, under 10.