"I am a big boy. I can handle it."

I listened to two of my coworkers talking. One, an American guy, asked the other, a Japanese woman, the direction to the place to visit. She answered, but he wasn't satisfied with his explanation. And then, he said, "I am a big boy. I can handle it."

It cracked me up because he called himself a boy, which originally means a "male child." I don't know whether it would sound to native speakers, but "I am a big boy. I can handle it" should be used by a 10-year-old school boy who wants to go somewhere without his parent's protection.