In Charlotte Brontë’s famous novel Jane Eyre, Jane is around nineteen years old and Mr. Rochester is described as being between thirty-five and forty years old. When Jane first encounters him, she describes him as a man who is "past youth, but had not reached middle-age; perhaps he might be thirty-five." Later, Mrs. Fairfax describes Mr. Rochester as being "nearly forty." This makes the age difference between the two of them about twenty years or so. This large age gap has made their relationship controversial among many modern readers, but there are several logical reasons for Brontë to have made the gap in their ages so large.
One point to consider is that Rochester has been married before Jane. His first wife, Bertha Mason, has been locked in the attic at Thornfield Hall for ten years. Thus, given that he's already had a significant life experience when he meets Jane, it makes sense that he would be much older than her.
It was also much more common in this time period than it is today for women Jane’s age to get married. There were not many viable economic opportunities that would allow women to remain independent, and it was typically expected that religious women would marry when they hit adulthood. That said, the gulf in their ages, not to mention their social statuses, would likely have raised some eyebrows, even for the time. As the host of the local inn recounts,
Mr. Rochester was about forty, and this governess not twenty; and you see, when gentlemen of his age fall in love with girls, they are often like as if they were bewitched.
When Mrs. Fairfax discusses the possibility of Blanche marrying Mr. Rochester, she mentions that there is a "considerable difference in age: Mr. Rochester is nearly forty; she is but twenty-five" and expresses some skepticism that Mr. Rochester would consider the match for this reason. Of course, not yet twenty, Jane is several years younger than Blanche, which suggests that it is Blanche's character, rather than her youth, which Mr. Rochester finds unappealing.