I think that her freindship with Kitty probably puts her as being around 18-20 or so... I knwo that Eliz is freinds with the much older Charlotte, but Eliz is highly intelligent and probalby always enjoyed the compnay of friends older than herself. (and she also has Jane as a companion who is only a little older than herself).

but Maria's nervousness in company plus her being it seems best mates with Kitty woudl seem to me to indicate that she's quite young. Probably, Charlotte is the eldest Lucas Child, and there may be a boy or 2 in between her and Maria, who are - I dont know at college, or in soeme genteel posistion or even genteelly idle like their Papa....
so these boys might be the ones who are relieved to think that Charlotte wont' be an old maid, while the girls are the ones younger htan Maria who are hoping to come out a bit sooner now that Charlotte is off their mother's hands... They might be say 16 and 17 or so, and are hoping that they will be allowed out now.. as Lady L probalby didn't want to have too many girls out at the same time...
whereas the youngest Lucas is problaby the one who disputes with Mrs Bennet about drinking - he sounds about 10 or 12....
so IMO the Lucases are probalby Charlotte, 2 sons, Maria, 2 more daughters and an afterthought boy.