A salesman is at the door of a family house and tries to convince the mother to buy some of his products. She says: "Well, I don't really need the products, but if you can guess how old my three daughters are I will do you a favor and buy one of them." The salesman asks the woman for a little help and she tells him: "The product of their ages is 36." As the salesman is not able to figure out the ages, she gives him another clue: "The sum of their ages is same as the number of my house (13)." The salesman calculates for a little while but then says: "I'm sorry, Miss, but I still can't figure out their ages." The mother agrees to give him a final hint: "My eldest daughter plays piano." After that hint the salesman was able to tell the mother her daughters ages and sell one of his items.
How did he figure that out?