Okay, it's not quite Doogie Howser or The Kid With The 200 I.Q., but it is real: Zachary Young, a 15-year-old sophomore at Stuyvesant High School, was accepted to Harvard, MIT and Caltech. He tells the Post that when he got the nod from MIT, "That was much more exciting [than Caltech's acceptance]. I had my mom, my sisters and my girlfriend, and they all screamed." (Yes, he has a girlfriend!) Zach, a huge math whiz who has already taken all of Stuy's math and science classes, doesn't watch TV or listen to music, "If I have a choice between math and almost anything else, I choose math." Maybe he won't have that girlfriend for long.