How old am I?

A kid goes up to his father and says, "Hey, Pop, know how old I am today?" His father says, "No...how old?" He says, "I'm eleven!" He goes into the kitchen and says to his grandmother, "Hey, Grandma, know how old I am today?" She says, "Come closer..." She unzips his jeans and reaches her thin, spotted arm down into his underwear. She fondles his genitals for a few minutes and then she says, "You're eleven." He says, "How could you tell?" She says, "I heard you tell your father."