Now standing almost 6-foot-6 and weighing close to 220 pounds, they have grown into one of the most talented sets of twins to ever come into college basketball, and the Kentucky Wildcats cannot wait to unleash them on opponents.
But first, the Big Blue Nation – and maybe even their own coach – has to learn what sets them apart.
Aaron comes to UK as the nation's top-rated shooting guard; Andrew as the nation's elite point guard prospect. They may look identical, but they also have distinct personalities, and those are often evident in the way the two siblings interact with each other on the court.
"They're very critical of themselves, and I'm going to tell you, each other," UK head coach John Calipari said. "So they do some yapping to each other."
Most of it, they admit, is constructive criticism. And it began at a young age.
After growing up learning each other's tendencies, the twins hold one another to the highest standard on and off the court. Sometimes it's misread by opponents or onlookers who aren't that familiar with them.
"If we weren't as critical we probably wouldn't be as good as we are today," Aaron said. "We don't let each other get away with anything. We know when the other is not going 100 percent, or we see a mistake being made that shouldn't be made, we let each other know about it."
The Harrisons refer to themselves as perfectionists. And that all started with strong rooted values inherited from their parents while growing up in the suburbs of Richmond, Texas.
"It's definitely the way we were raised," Aaron said. "My parents didn't let us get away with much when we were younger, and they were always tough on us about the little things because that's what matters."
Andrew comes into this season with the added pressure of not only being the de facto leader of a young squad loaded with talent, but also attempting to restore UK's reputation as "Point Guard U" under Calipari.
After Kentucky's flawed experiment at the point in 2012-13, Andrew hopes to jump back in line with the rest of Calipari's phenomenal lead guards of the past. While he draws comparisons to players like Derek Rose, John Wall and Brandon Knight, Andrew remains humble and hungry to do one thing – win.
"Being compared to a lot of those guys is great, but at the same time I have a lot of work to do to be on those guys' level. I have to be in the gym, and we have to win," Andrew Harrison said. "That's the most important thing is winning."