Well, since you ask... ;-)

Jane is 22, and at the end of ch. 1, it says that "the experience of three-and-twenty years had been insufficient to make [Mrs. Bennet] understand [Mr. Bennet's] character", so presumably they have been married 23 years, or a little over. I always assumed that Mrs. Bennet was young, but not a teenager, when she married - say age 20, and that Mr. Bennet was at least a few years older - say 25. (No direct evidence in the book for this, but it makes sense to me!) That would make them about 43 and 48 respectively when the novel opens. To be still in her late 30s at this point, Mrs. Bennet would have had to be 16 at most when she married, which even in those days was considered very young, as we see with Lydia and Georgiana.