An elderly man was having a calm walk when he happens to pass a brothel.

One of the prostitutes calls out: "Hey Grandpa!  Why don't you try ?

The old man replies: "No, my child, I cannot!"

The prostitute: "Cheer up !!! Let us try !!!"

The elderly man enters and performs like a 25 year old

The prostitute says:"Oh Gosh! And you still say you cannot" 

The old man replies:  - "Aaah, sex I can, what I cannot is pay!"