The Wife of Bath is an old woman. We know that she's past forty, and back in Chaucer's day, people didn't live much longer than that on average. The only effect this seems to have had on the Wife, however, is that some of her youthful beauty is gone and it's getting harder for her to appear desirable to men. Old age has not dampened her appetite for sex. On the other hand, it seems to have had that effect on all the men her age, who have trouble keeping up with her sexually; perhaps for this reason the Wife has a "coltes tooth," preferring much younger men as sexual partners.
Beyond associating age with a loss of sexual vigor in men, the Wife also links it to a wisdom gained from life experience. She often refers to her numerous years to support the arguments she makes drawing on this experience. To sum up: old age in the Wife of Bath's Prologue = loss of beauty in women, low libido in men, and lots practical wisdom. But also, and perhaps most importantly, it leaves the Wife in a nostalgic frame of mind, and she enjoys reminiscing about her youth and past experiences. And it is her telling of these past experiences that makes up a good portion of the Wife of Bath's Prologue.
The Wife of Bath's advancing years are a significant part of her argumentative strategy because of her claim to speak from life experience.
The Wife of Bath manages to seem both young and old by referring to her advancing years while at the same time professing an unabated sexual appetite and making use of numerous flashbacks.