She may have married very young, and she doesn't seem to have much interest in anything besides gossip and getting her girls married. We see some of her childishness in her teasing argument with the young Lucas boy and in her advising Lizzy not to dance with Darcy if he ever does ask her. Instead of admonishing Lydia about her behavior with the officers, Mrs. Bennet recalls a time when she too pined after the redcoats. And then when Lydia runs off, Mrs. Bennet blames everyone but herself, takes to her bed and requires constant attention at a time when her husband and Lizzy are both away and Jane has been left to handle her father's correspondence in an extremely sensitive matter. She's not acting like a mother here but like a petulant 8-year-old.