That was the by-product of being sons of a rugby league great, Australia and NSW State of Origin full-back Garry Jack.
But Kieren turned his attention to Australian football in his teens and his youngest brother Brandon has done the same.
Due to his famous father, Kieren drew plenty of attention when he made it onto the Swans list in 2007, but he has since become one of their most important players, winning the best and fairest in 2010.
Renowned for his toughness and ability to win the ball, Jack said his youngest brother Brandon, who made his debut with the Swans reserves this season, is a very different type of player.
"He's a different player to me purely based on his size and build," Kieren said.
"He's probably about six foot already and he's very skinny, very raw in his ability, but he's a really athletic type.
"He's a runner and a jumper and all the things that I'm not. I'm in-and-under, get the ball out of packs [type]. He tries to do that, only because he watches me play on the weekends and comes to all the games.
"But I'll be telling him to really work on his outside pace because he's explosive and can run and mark and jump.
"He's a real wingman, half-back, half-forward type of player."
Having not missed a game over the past two seasons, Kieren Jack has twice been struck down by ankle injuries this season, missing six games.
It hasn't allowed him to produce his best football, but he stood out in the impressive win over the Western Bulldogs in their last match and is a huge key to the Swans' chances of success this season.
"It's been different this year. I've obviously missed a few games, which I'd never previously done," Jack said.
"It probably made me appreciate being fit and being healthy and able to play each week because there's nothing worse than sitting on the sidelines and not being out there with the boys.
"But I'm confident now that it's all in the past and against the Bulldogs it was a good step forward and I got a lot of confidence out of it heading into what will be a really important five-week block for us."
Throughout 2011, Swans coach John Longmire has stressed how important it is for both Jack and ruckman Shane Mumford to be in the side.
But Mumford has also been in and out of the side, missing five games through injury and suspension.
Both are now back and Jack relishes the combination with Mumford, who finished runner-up in the best and fairest in his first year at the Swans.
"I think myself and 'Mummy' work really well together and we've got a good understanding," Jack said.
"We get along well off the field and work together well on the field and it was certainly frustrating for him missing games through injury and suspension.
"He's such a great tap ruckman and his follow-up work and the presence he has on the ground is enormous, especially for a small bloke like me, he can look after me a bit.
"So it's great that we're back out there together and helping the rest of the midfield unit."
Jack might need Mumford's protection if his brother Brandon continues to develop.
Kieren said his youngest brother used to cop the worst of those backyard tussles and might come looking for retribution.
"I've got two younger brothers and the other one's about to turn 22 and about to start playing NSW Cup with the Balmain Tigers in the NRL," he said.
"So growing up it was very much rugby league out in the backyard, it was tackle footy and it was full on.
"Brandon's the youngest and he used to cop a battering because when we were about 15, he was about seven or eight.
"There used to be a few tempers flared and mum would blow up at us for hurting little Brandon.
"He copped it the most and he's already the tallest, he'll be the biggest out of all three of us. I'm just waiting for payback."
Kieren doesn't know if Brandon can make it to the AFL, but says he does possess the required work ethic and hunger.
It's the same sort of commitment the sixth-placed Swans need if they are to have a successful season, starting with Saturday night's clash against Essendon at Etihad Stadium.
"Our season was on the line no doubt against the Bulldogs and we produced probably our best four-quarter effort for the year," Jack said.
"Things can't change this week, we're certainly under the pump and there's probably about five or six teams fighting for the last four spots on the ladder and we can't afford to slip up now.
"Every game's crucial and this weekend can be a real season-defining game for us."
