Ahrens attended Memorial High School in Hedwig Village, Texas. At the 2006 World Wood Bat Championships, he led the Houston Heat with game-winning hits in the quarter- and semifinals. He was named player of the year in the Greater Houston area and was twice selected all-state and three times all-district.
Prior to the 2007 draft, he signed to play baseball with Texas A&M University. Sports Illustrated projected Ahrens as a top-20 prospect for the 2007 draft.
After being selected by Toronto in the sixteenth spot, Ahrens decided to forgo college and enter the Blue Jays organization. He was given a $1.44 million US signing bonus, the 10th-highest for a Blue Jay prospect, which included $60,000 for schooling to offset his not going to Texas A&M University.