Zachary Young’s acceptance letters to Harvard, MIT and Caltech arrived early at his Upper East Side apartment last month.

Two years early.

Not even having finished his sophomore year at the city’s elite, public Stuyvesant HS, the 15-year-old wunderkind has already secured seats for himself at three of the country’s brainiest colleges.

For a while, it was a mailbox flurry of good news.

“I was pretty shocked. I hadn’t really pictured myself getting a letter,” the brainiac said of his first acceptance — from Caltech.

But it was the MIT green-light that shot him over the moon.

“That was much more exciting,” he gushed. “I had my mom, my sisters and my girlfriend, and they all screamed.”

Young says he still hasn’t decided which university to attend.

“I’m still totally not sure,” he admits. “I flip-flop like every day.”

Young’s aptitude was obvious from preschool, although he also once read an “Idiot’s Guide to World War II” — when he was in third grade.

His passion for math began at age 2, when he finished a puzzle — and then did it upside down to make it more challenging.

“He’s very intense and driven,” said his mom, Lisa Young, who works in the office at the Anderson School for the intellectually gifted on the Upper West Side. “My only reservation is that I wish he could know that he’s only going to be a kid once. He’ll have plenty of time to be a grown-up.”

But Young is impatient, especially with those folks who just don’t get math.

“A lot of people have the misconception that it’s plugging and chugging,” he said. “That’s not real math. Real math is basically proving things and solving problems.”

Young has already taken every math and science class Stuyvesant has to offer.

“I wasn’t sure what classes I’d be able to take next year,” said Zach, who attended the private Ethical Culture Fieldston School through fifth grade before switching to the Anderson School in sixth. “I really just applied to college to give myself options.”

Zach is a two-time national chess champion who doesn’t watch TV or listen to music.

“If I have a choice between math and almost anything else, I choose math,” he said.

yoav.gonen@nypost.com

