Complex sentence: Higginson, a wealthy planter, was expected to act as her guardian until Elizabeth Key reached the age of 15, considered the "coming of age" for girls, who frequently married that young or started work for wages.
Split into simpler sentences: Higginson, a wealthy planter, was expected to act as her guardian until Elizabeth Key reached the age of 15, considered the "coming of age" for girls. They frequently married that young or started work for wages.