Slack, who led Australia in 19 Tests, including a breakthrough away series win over New Zealand in 1986, said Steve Hansen's team was not the formidable outfit that swept to
World Cup glory in 2011, and could be tested in a battle of fitness by the more youthful Wallabies.
"Depending on which 15 or 23 they will pick, there are a lot great players there in terms of the Carters, the McCaws andthe Smiths, but they're also 31, 32 years-old now," Slack told Reuters in an interview on Monday.
"The Australians will look at that and push it to them in the closing minutes, and see whether they're hungry enough.
"As much as they'd hate to admit it, age does become an issue and it will be a factor."
Australia and New Zealand clash in Sydney on Saturday, the opening match of the southern hemisphere Rugby Championship, which also includes South Africa and Argentina.
New Zealand's 28-man squad for the tournament is led by 32-year-old skipper Richie McCaw, one of eight players over 30.
Flanker McCaw, whose captaincy has coincided with an extended period of dominance over Australia, recently returned from a six-month sabbatical and queries over his fitness remain after he appeared underdone in a practice match on Friday.
Hansen has backed McCaw to be fit for Sydney and is likely to put his faith in other 30-something veterans, including fly-half Dan Carter, centre Conrad Smith and hooker Andrew Hore.
In hooker Stephen Moore, new Wallabies coach Ewen McKenzie has only a single player above 30 in his squad, with most in their early-to-mid 20's.
Already conceding a wealth of experience, Australia head into the tournament opener on the back of a demoralising series-deciding loss to the British and Irish Lions, which pre-empted former coach Robbie Deans's sacking.
New Zealand, however, belted France 3-0 in their June internationals and have signalled their intent to gradually regenerate by omitting experienced scrumhalf Piri Weepu and loose forward Victor Vito.
McKenzie's new broom has nonetheless given the Wallabies renewed hope of ending New Zealand's dominance and winning back the Bledisloe Cup, the annual symbol of trans-Tasman supremacy, for the first time in a decade.
The All Blacks have played down the 'McKenzie factor', but the Wallabies would be primed to make the most of a clean slate after having lost 14 out of 18 against the All Blacks under New Zealander Deans, Slack said.
"I think the change of coach and regime will help in the psychological side of things. There will be a freshness mentally ... It won't be a matter of 'here we go again'," he added.
"They didn't win against the Lions but a few guys made a bit of a name for themselves, like (inside centre) Christian Leali'ifano, and they'll go into the series confident."
One of the players looking for a clean break from the past will be fly-half Quade Cooper, who has struggled against the country of his birth, but is expected to reassume the Wallabies' fly-half position under McKenzie, his former coach at the Queensland Reds, after a near year-long exile.
Cooper blamed Deans for his struggles to emulate his brilliant provincial form at Test level in an extraordinary attack last year, and Slack agreed that the 25-year-old was stifled by the New Zealander's conservative gameplan.
"I think he's in a position now that there's no more excuses on that front," Slack said of the mercurial playmaker.
"He's been round long enough and I think under McKenzie, knowing each other so well now, we'll see the best of him.
"Like any footballer, if he doesn't perform, there's only one way to go.
"If he's picked for every one of the All Blacks Tests and he doesn't play well and we don't win, then he nor anyone else would expect he would keep getting picked."