Usually the prepubescent years (before age 13) is when kids discover who they are and what they like doing. It's something that can't be taught. It's more of a feeling; for example, liking music or auto mechanics. Then, the teenager starts gravitating towards these interests sometimes unconsciously. That's not to say they can pursue their life's calling. Artists, for example, have always struggled to make a living and end up doing something else to make a living.
I believe that our life's calling is determined in the womb where we hear sounds and feel vibrations which helps determine the personality. Also, many people feel inner confict. I think that's because they are doing a job that is going against their inner being; for example, a concert pianist who is forced to make a living as a plumber.
Knowing who you are takes great introspection. And don't be afraid to pursue your dream. To not do so is to throw away your life.