I cannot

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!"