Mark Taylor was four years away from his Test debut when Allan Border first assumed the Test captaincy. He was still a season away from his first class debut.
In fact David Boon was the only player to play Test Cricket under the captaincy of both Kim Hughes and Mark Taylor whilst Craig McDermott was the only other player to play in a Test alongside Hughes and under Taylor. Dean Jones and Greg Matthews were captained by Hughes in Tests & ODI's and by Taylor in just ODI's.
Steve Waugh captained Australia in Test Cricket before Adam Gilchrist made his Test debut and when S.Waugh was unavailable for a Test it was Gilchrist who took over the captaincy.
Ricky Ponting was Test captain before Michael Clarke played Test Cricket. In fact Clarke's Test debut came when Ponting was unavailable due to injury.
It's not that far fetched the next long-term captain of the Australian Test team is currently not in the side.
Smith has plenty of leadership credentials already at age 23. He doesn't yet have the game to have a substantial Test career, but he does have time on his side and if he gets his game in complete order will have experience on his side.
I remember after the 1989 Ashes Tour a lot of experts were asked to name their 1993 side. All of them had Taylor and Steve Waugh in the side, only half had Allan Border, they all had Mark Waugh in the side, most had Craig McDermott making a successful comeback, most had the no.2 spinner Tim May in the side and all believe it or not had Tim Zoehrer in the squad. But not one squad list had Michael Slater, Shane Warne or Paul Reiffel - all three strong performers on that Ashes Tour.
Steve Smith isn't that far fetched. If his game matures with his age and he warrants a Test spot he will be the most qualified option as Test captain...