When you come out, you need to tell enough of the story so they understand how you came to the decision but you need to tell only what you are comfortable with. I now understand that while I understood at age 13, I showed signs dating back to early childhood. When you think about it, you may also have something like that in your past that would fill out the storyline so they don't probe to deep into the moment of discovery. The more information you can provide them with, the less they will ask.