When I was 11, children in my boarding school sought me out for comfort and to answer their questions about life. They would come to my room at night and I would speak with them to soothe their fears and loneliness.
When I was 12, I began to send away for spiritual books, such as the Tao Te Ching, The Way of Zen, The Bhagavad Gita, and the works of philosophers. I lived in a small town that did not have a sufficient library. I bought these books with my own money and was not encouraged by my parents, who sought answers from our family religion.
At 16 I was mostly interested in the spiritual developments in the world and, by request, started an informal counseling practice after school. Students made appointments with me to discuss their problems and to find answers that would lead to peace in their hearts.
In my case then, I knew very early what my chosen field would be. For others, it takes a while before they understand their calling. They may not be able to practice their true calling right away but it is unavoidable if it is really what they are meant to do in life. They will do it whether it brings an income or not. It is something they do because they love it, they have to do it, and it is inevitable.