I think we have to be careful about the difference between women's age at marriage in JA's *time* vs. in JA's *fictional world*. ;-) You're right that in JA's novels, her main heroines seem to marry pretty early in life - late teens and very early 20s. Even Anne Elliot would originally have married at 19 or 20 if she could.

However, I was surprised to find during our GR of "The Gentleman's Daughter", that the *mean* age for women to marry during the Georgian age was actually 24 years and 9 months. Also, Anielka posted a fascinating list at the L&T board back in December about the ages JA's immediate female relatives were when they first married (linked below). So, if you go by the realities of the time, Isabella has no need to worry yet, but by the rules of JA's fictional world, perhaps she does. ;-) (Also, of course you may be right that she is worrying about her financial situation.)