פירוש על קידושין 81:8

The mishnah states that a man may marry off his daughter when she is a na’arah (has reached the age of puberty, but is not yet an adult). From here the Talmud concludes that he may not marry her off when she is minor, until she is old enough to say that she wants a certain man. While we have a tough time imagining a 12 year old girl making such a statement, it seems that in their society this was not so outlandish.