Mary is between 18 and 20, based on the fact that Kitty is two years older than Lydia, and Elizabeth is less than 21 years old. Kitty and Mary could possibly be fraternal twins, something that would have given Mrs. Bennet a longer rest between pregnancies, though is properly not what Austen had in mind for them. She is much more sensible than Kitty and Lydia Bennet, but Mary is still often considered to be silly by her father. She is studious, and apparently the most accomplished of the Bennet sisters (she is actually described as the most accomplished girl in the neighborhood at one point), but unfortunately she lacks genius and taste and has too much pedantry and conceit. She is
probably the most ignored of the Bennet daughters, which may be why she puts so much effort in trying to impress people. Apparently after her sisters went away, Mary also improved quite a bit as she was finally given a consistent amount of her mother's attention.
In James Edward Austen-Leigh's A Memoir of Jane Austen, Mary ended up marry One of her Uncle Phillips's law clerks (an apprentice lawyer, according to James Edward Austen-Leigh). Afterwords she moved to Meryton with him.