What law forces Llewelyn's mother, Marared, to allow her son to remain in Wales and reclaim his father's lands?

Marared is forced to allow Llewelyn to remain in Wales after attending the funeral of her brother Owain because of a Welsh law that states that a boy should be recognized as a man at the age of fourteen. As long as Llewelyn is on Welsh soil he is considered a man and able to leave his family without their consent, where in England he would remain a ward of Hugh Corbet until his twenty-first birthday.