How old was Alisoun of Bath when she first married?
The answer to this can be found in The Wife of Bath's Prologue. She does not exactly come out and say "I got married when I was..." at least she does not in the translation of the book that I have. But she does imply that she first got married when she was only 12 years old.
She implies this by saying that she has found that marriage is a "misery and a woe." She says that she has known this since she was 12 years old. She has been married five times overall.