Mysterious Age Guessing

It's this man's 33rd birthday. He gets a package at the Post Office and goes to collect it. At the counter the woman brings his package to him, and the man says, "It's my birthday today." "Oh, happy birthday, how old are you?," asks the Post Office worker. "33," says the man. "Well, have a good day," says the worker. "Thank you," replied the man. To get home, the man has to take the bus. At the bus stop an old lady walks up and waits soon after he arrives. The man says to the old lady, "It's my birthday today." "Oh, happy birthday," says the old lady. "I'm..." "No don't tell me," interjects the old lady, "I know a unique way of telling how old somebody is." "Oh yeah? What's that then," asks the man. "If I can feel your balls for about 5 minutes, I can tell exactly how many years old you are," says the old lady. "I don't believe it." "Well let me prove it!" "I'm not going to let you feel my balls!," says the man. "Oh well, I guess you'll never know then," replies the lady. After a couple of minutes curiosity gets the better of the man and he says, "Oh, okay then, you can do it." After a good feel of the man's balls the woman finally takes her hands out of his pants. "You are 33 years old exactly," she exclaims! "How the heck did you know that?!," exclaims the man, impressed. "I was behind you in the line at the Post Office," said the lady.