Although Mother Teresa became a nun at 18 years old, it wasn't until she was 38 that she left her convent and opened a school in the slums of Calcutta [source: Biography].
For 17 years she taught at St. Mary's School for Girls in India. During that time she experienced what she referred to as "the call within the call" to work with the poor and the suffering she saw in the streets of India. At 38, she left the Sisters of Loreto to move into the slums where she started a school, and also tended to the sick and the dying. After a few months, she was joined by other volunteers, many of them former students [source: MotherTeresa.org].
In 1950 at age 40 she established the "Missionaries of Charity" order with 13 nuns. This eventually became a world-wide organization that helps the sick, the poor, the dying, as well as disaster victims through a network of religious houses, schools, hospices, and charity centers in more than 120 countries [source : The Nobel Foundation].