A prostitute standing outside a motel in a small town saw a 70+ year old man walking past. She hasn't had a customer for a while so she whistles at him and says, "Hey, would you like to have some fun time with me?"
The old man said, "But I won't be able to..."
"C'mon man.... give it a try... " She says.
Old man says okay. They go in. The moment they get to the bed, the old timer becomes a machine and makes passionate love to her an hour straight.
When he's done, the prostitute catches her breath. Exhausted and tired she says, "But you said you won't be able to...."
"...pay you." completed the old man.