How old is Mattie and why is she "indentured" to Ethan and Zeena?

Mattie was twenty years old, and her father was Zeena's cousin. He had left the hills and traveled to Connecticut, but he died too young to make his fortune. His wife died soon after, leaving Mattie to fend for herself. She had no real skills, and the difficulties of working for a living disagreed with her health. When Zeena's doctor recommended that Zeena find a housekeeper, the family arranged to have Mattie stay at the Frome farm. Although life there has been harder than her old life in Conneticut, Mattie has been cheerful and happy. However, she has little talent for housekeeping, and her ineffectiveness vexes Zeena.