Joke: The 13-Year-Old

A man notices his thirteen-year-old son has been walking around for a week with a hard-on in his pants, and figures the boy needs some relief. He gives the kid twenty dollars and says: "Take this to the whorehouse at the other end of town, and have some fun."
Well, the whorehouse is at least a good hour’s walk each way, but the boy returns in only 45 minutes. The father says: "Where were you?"
"Well," says the boy, "when I walked by Grandma's house she asked where I was going and I told her. She said, 'Why don't you just come in here instead, and I'll do you for free.' So that's what I did."
The father, outraged, screams: "Are you telling me you fucked my MOTHER?"
"What's the big deal," says the boy, "You fuck mine!"