Well, you always "KNOW" from a very early age. It is just that until you get older you don't "KNOW" what the "KNOWING" is all about. "KNOW" what I mean?
You notice subtle differences growing up between you and the other kids of the same gender. For instance, when you are in kindergarten, all the other little boys don't have an inkling of clue in regards to coordinating Barbies purse and shoes. Their outfit selection just looks horrendous . Even then, the opposite sex still elicits EEWWWWWW from all male toddlers. Then by first grade, you notice your teacher stealing your Lite Brite creations and entering them in the County Art Fair. And winning. By third grade, you already know the Churches are hypocritical and if you are going to hell (along with the rest of those morons) you are going to put your Sundays to better use. Like getting a tan. By fifth grade, you are several years intellectually advanced than your classmates. You know geography better than anyone else, because you have already planned your future vacations in half the world. By seventh grade, you actually read National Geographic for the articles... not to see the sagging tits on some ugly woman in the middle of God Forsaken Africa. (A place that failed to make your vacation list). Entering high school in ninth grade, when taking Home Ec. classes... your whites are whiter than anyone else's. Your cake NEVER falls. Or fails to be served at the PTA meetings. You are the only 9th grader in HISTORY to be invited to the Prom. You go. All the girls line up to dance with you because... you can actually dance. Never would their Prom pictures look so good without you there to stand in the place of Bubba, Red, and Johnny. 11th Grade... you are actually in the Prom. Again. Third year in a row. Same old shit. Same ol' looking the best of all the rest. You also notice.. again... that everyone there is wearing what you had on LAST YEAR! Puh-Leaze! 12th grade... you are graduating... unlike some of the other people in your class... the biggest worry this year is not whether you will get that C in spanish you need to graduate, but whether you are picking up that GPA schloraship at Yale, or riding the atheletic scholarship through various colleges who are still calling by the week.
College.. real life starts now.
To be honest, I am not exactly one to answer the question of "when did I know" I banged so many girls through high school and college it was not funny. Somewhere along the lines, I started banging guys and for a while there was a balance, but then the balance shifted to an absolute level of 100% guys, 0% women.